Is there any way of changing the numbers in a league when it is up and running. I am not host of a league and although the Elite is 32 strong the Novice and Pro are not. Same with many league, struggling to fill the lower divisions.
Once a league has been created, there is no way to switch is from a 1 car league to a 2 car league or vice versa.
However you could try making a new, 1 car league and persuade all the current managers the new league, however, there is a chance that they won't want to.
*NOTE - If you leave the league you host, anyone who has been in the league for 14 days can take over without your prior notice*