Question: How many teams were represented in the pair of motor races held on 18 and 19 April 2015 at the Bahrain International Circuit in Sakhir, Bahrain?

full_context: Document 1: 2015 Bahrain Grand Prix: The 2015 Bahrain Grand Prix (formally known as the 2015 Formula 1 Gulf Air Bahrain Grand Prix) was a Formula One motor race that was held on 19 April 2015 at the Bahrain International Circuit in Sakhir, Bahrain. The race was the fourth round of the 2015 season and marked the eleventh time that the Bahrain Grand Prix has been run as a round of the Formula One World Championship.
Document 2: 2016 Baku GP2 Series round: The 2016 Baku GP2 Series round was a pair of motor races held on 18 and 19 June 2016 at the Baku City Circuit in Azerbaijan as part of the GP2 Series. It was the third round of the 2016 GP2 Series and was run in support of the 2016 European Grand Prix. The first race, a 26-lap feature event, was won by Prema Racing driver Antonio Giovinazzi who started from pole position. Sergey Sirotkin finished second for ART Grand Prix, and Russian Time driver Raffaele Marciello came in third. Giovinazzi won the second event, a 21-lap sprint race, ahead of teammate Pierre Gasly in second and Sirotkin in third.
Document 3: 2013 Bahrain GP2 Series round: The 2013 Bahrain GP2 Series round is a pair of motor races held on March 23 and 24, 2013 at Sepang International Circuit, Malaysia as part of the GP2 Series. It is the second round of the 2013 season. The race weekend supported the 2013 Bahrain Grand Prix.
Document 4: 2012 Bahrain GP2 Series rounds: The 2012 Bahrain GP2 Series rounds were a series of GP2 Series motor races held at the Bahrain International Circuit as part of the 2012 GP2 Series season. The first round was held on 21 and 22 April, in support of the 2012 Bahrain Grand Prix. The second round was held one week later as an independent round of the championship, the only one of its kind on the 2012 calendar.
Document 5: 2015 Red Bull Ring GP2 and GP3 Series rounds: The 2015 Red Bull Ring GP2 and GP3 Series rounds was a pair of motor races held on 20 and 21 June 2015 at the Red Bull Ring in Austria as part of the GP2 Series and GP3 Series. It is the first round of the 2015 GP2 season and 2015 GP3 season. The race weekend supported the 2015 Austrian Grand Prix.
Document 6: 2009–10 GP2 Asia Series: The 2009–10 GP2 Asia Series season was the third season of the GP2 Asia Series. It began on 31 October 2009 and ended on 14 March 2010 after four double-header rounds. All teams from the 2009 GP2 Series season except for Racing Engineering were due to take part, with the thirteenth slot being filled by MalaysiaQi-Meritus.com. Durango withdrew from the season for financial reasons. The season was the last to use the first specification of the GP2 car, as used in Europe between 2005 and 2007.
Document 7: 2012 Marina Bay GP2 Series round: The 2012 Marina Bay GP2 Series round is a group of motor races that will be held at the Marina Bay Street Circuit in Marina Bay, Singapore for the GP2 Series championship, with the 2012 GP3 Series season having concluded at the previous round at Monza. The races, which will be held on 22 and 23 July, will be in support of the Singapore Formula One Grand Prix, and represent the final round of the 2012 GP2 Series season. The event marks the first time the GP2 Series has travelled to Singapore.
Document 8: 2014 Bahrain GP2 Series round: The 2014 Bahrain GP2 Series round was a pair of motor races held on 6 and 7 April 2014 at the Bahrain International Circuit in Sakhir, Bahrain as part of the GP2 Series. It was the first round of the 2014 GP2 Series and was run in support of the 2014 Bahrain Grand Prix. The first race, a 32-lap feature event, was won by ART Grand Prix driver Stoffel Vandoorne after starting from second position. Julián Leal finished second for the Carlin team and DAMS driver Jolyon Palmer came in third. Palmer won the second race, a 23-lap sprint event, ahead of Rapax driver Simon Trummer in second and Leal third.
Document 9: 2015 FIA Formula One World Championship: The 2015 FIA Formula One World Championship was the 69th season of FIA Formula One motor racing. It featured the 2015 Formula One World Championship, a motor racing championship for Formula One cars, recognised by the sport's governing body, the Fédération Internationale de l'Automobile (FIA), as the highest class of competition for open-wheel racing cars. Twenty-two drivers representing ten teams contested nineteen Grands Prix, starting in Australia on 15 March and ending in Abu Dhabi on 29 November as they competed for the World Drivers' and World Constructors' championships.
Document 10: 2015 GP3 Series: The 2015 GP3 Series was the sixth season of the GP3 Series, a motor racing feeder series that was run in support of the FIA Formula One World Championship and sister series GP2. Thirty-one drivers represented eight teams over the course of the season's nine rounds, starting in Spain on 9 May and ending in Abu Dhabi on 29 November. It was the final season for the Dallara GP3/13-AER P57 V6 chassis and engine package that debuted in 2013. A new Dallara-built chassis was introduced for 2016 along with a Mecachrome normally-aspirated V6 engine.
Document 11: 2015 Bahrain GP2 Series round: The 2015 Bahrain GP2 Series round was a pair of motor races held on 18 and 19 April 2015 at the Bahrain International Circuit in Sakhir, Bahrain as part of the GP2 Series. It was the first round of the 2015 GP2 Series and was run in support of the 2015 Bahrain Grand Prix. The first race, a 32-lap feature event, was won by ART Grand Prix driver Stoffel Vandoorne who started from the pole position. Rio Haryanto finished second for the Campos Racing team, and Racing Engineering driver Alexander Rossi came third. Haryanto won the second event, a 23-lap sprint race, ahead of Vandoorne in second, and Lazarus driver Nathanaël Berthon in third.
Document 12: 2017 Bahrain FIA Formula 2 round: The 2017 Bahrain FIA Formula 2 round were a pair of motor races held on 15 and 16 April 2017 at the Bahrain International Circuit in Sakhir, Bahrain as part of the FIA Formula 2 Championship. It was the first round of the 2017 FIA Formula 2 Championship and was run in support of the 2017 Bahrain Grand Prix.
Document 13: 2005 Bahrain GP2 Series round: The 2005 Bahrain GP2 Series round was a GP2 Series motor race held on September 29 and 30, 2005 at the Bahrain International Circuit in Sakhir, Bahrain. It was the final round of the 2005 GP2 Series season. The race weekend was a stand-alone event unlike the previous GP2 rounds which support Formula One Grands Prix.
Document 14: 2015 Monaco GP2 Series round: The 2015 Monaco GP2 Series round was a pair of motor races held on 22 and 23 May 2015 at the Circuit de Monaco, Monte Carlo, Monaco as part of the GP2 Series. It was the third round of the 2015 GP2 season and was run in support of the 2015 Monaco Grand Prix. The first race, a 40-lap feature event, was won by ART Grand Prix driver Stoffel Vandoorne who started from fourth position. Alexander Rossi finished second for Racing Engineering, and MP Motorsport driver Sergio Canamasas came in third. Status Grand Prix driver Richie Stanaway won the second event, a 30-lap sprint race, ahead of Trident's Raffaele Marciello and Sergey Sirotkin of the Rapax team.
Document 15: 2015 GP2 Series: The 2015 GP2 Series season was the eleventh GP2 Series season, a championship for open-wheel racing cars run as a support series to the 2015 Formula One World Championship. Twenty-six drivers representing thirteen teams raced over eleven rounds, starting in Bahrain on 18 April and finishing in Abu Dhabi on 29 November.
ground_truth: thirteen teams