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?
Sentence (from "2015 GP2 Series"): Twenty-six drivers representing thirteen teams raced over eleven rounds, starting in Bahrain on 18 April and finishing in Abu Dhabi on 29 November.
Relevant evidence: Yes