The crowd is your club's fans who attend your match. Because you set the ticket price low enough, so the crowd is always at maximum of either stadium capacity or number of club's supporters, whichever is less.

Playing at home with full crowd, you gain 5% bonus on ball possession. Spectators are other managers who attend your match. For each spectator supporting your side, you gain another 2% bonus on ball possession. The maximum bonus on ball possession is 15%.