How many cars are there allowed on the starting grid at a F1 grand prix?

Each team is allowed to field two race cars per race. Every year the Formula One administration decides the number of teams that are allowed to compete every season. So, based on the number of teams, the number of cars differs. For ex: in the 2013 season, there are a total of 11 teams which means 22 cars will be allowed to be at the starting grid of each f1 grand prix this year.