Every seasonal race in each league is made up of 15 co-ops with the same amount of members with their Participate in Regatta status on.
If you join a co-op or activate the Participate in Regatta setting after the start of a race, you won't be able to complete tasks and help your co-op race. This is true for all leagues and co-ops so that it won't bring unfair advantage to some co-ops. Don't worry—this limit is only in effect until the start of the next regatta.
Please note that only those who have their Racing status on are counted towards the total number of participants. In other words, if your co-op has 20 racing members, it will be matched with the co-ops with the same numbers of racing players.