b-list is 100% unlimited, the main restriction is the age. any number of u21 players can be introduced on the b-list for any match, if those players have already played two years for the club between their age of 15 and 21.
44.10 Each club is entitled to register an unlimited number of players on List B during the season. The list must be submitted by no later than 24.00CET on the day before the match in question.
44.11 A player may be registered on List B if he is born on or after 1 January 1998 and has been eligible to play for the club concerned for any uninterrupted period of two years since his 15th birthday by the time he is registered with UEFA. Players aged 16 may be registered on List B if they have been registered with the participating club for the previous two years without interruption.