Every race in each league is made up of 15 teams with the same amount of members who checked the box "Racing".
Please note that only those who check the box "Racing" are counted towards the total number of participants. In other words, if your neighborhood has 10 racing members, it will be matched with the teams with the same number of racing players. It's quite possible that a team can have 20 members but only 10 are actually racing.
If you joined a neighborhood or checked the box "Racing" after the race started you won’t be able to complete tasks. Don't worry - this limit is only in effect until the start of the next race.