Just reading over at forzamotorsport.net forum and there is a post about the in game club limits.
100 club members.
200 shared club cars.
Link: http://forums.forzamotorsport.net/forums/thread/4925642.aspx
100 club members.
200 shared club cars.
Link: http://forums.forzamotorsport.net/forums/thread/4925642.aspx