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?

Context paragraphs:
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.

2017 Bahrain Grand Prix: The 2017 Bahrain Grand Prix (formally known as the 2017 Formula 1 Gulf Air Bahrain Grand Prix) was a Formula One motor race that took place on 16 April 2017 at the Bahrain International Circuit in Sakhir, Bahrain.  The race was the third round of the 2017 season and marked the thirteenth time that the Bahrain Grand Prix has been run as a round of the Formula One World Championship.  The winner of the previous Bahrain Grand Prix, Nico Rosberg, did not compete as he had retired after the 2016 season.  Before the race, Sebastian Vettel and Lewis Hamilton shared the lead in the Drivers' Championship.  Mercedes led the Constructors' Championship by one point over Ferrari.

2014 Bahrain Grand Prix: The 2014 Bahrain Grand Prix (formally the 2014 Formula 1 Gulf Air Bahrain Grand Prix) was a Formula One motor race held on 6 April at the Bahrain International Circuit in Sakhir, Bahrain.  It was the third round of the 2014 Formula One season, the 900th Formula One World Championship event, and the eleventh running of the race.  The 57-lap race was won by Mercedes driver Lewis Hamilton who started from second position.  His teammate Nico Rosberg finished second and Force India driver Sergio Pérez came in third.  It was Hamilton's second victory of the season, his first in Bahrain, and the 24th of his Formula One career.

2012 Bahrain Grand Prix: The 2012 Bahrain Grand Prix (formally known as the 2012 Formula 1 Gulf Air Bahrain Grand Prix) was a Formula One motor race held on 22 April 2012 at the Bahrain International Circuit in Sakhir, Bahrain.  It was the first time Formula One returned to Bahrain after the 2011 race was cancelled due to ongoing anti-government protests.  The race, the eighth running of the Bahrain Grand Prix, was contested over 57 laps and was the fourth round of the 2012 Formula One season.

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.

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.

Sakhir: Sakhir (Arabic: الصخير‎ ‎ ) is a desert area located in the Southern Governorate of the Kingdom of Bahrain, near Zallaq.  It contains the Al-Sakhir Palace, built in 1870.  In the early 2000s, the area has seen many changes such as the creation of the University of Bahrain main campus and Bahrain International Circuit, the home of Formula One's Bahrain Grand Prix.  In addition, many people come to Sakhir for camping.  Sakhir is also the location of the Al Areen Wildlife Park, the only nature reserve and zoo located on land in Bahrain.

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.

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.

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.

Supporting paragraphs: 2015 Bahrain GP2 Series round 2015 GP2 Series
Answer: thirteen teams