We'll use AFL as the worked example here. Other sports follow the same shape with different numbers. The differences that matter are at the end.
The shape of a squad
An AFL squad is 23 players: a field of 6 defenders, 6 midfielders, 1 ruck and 6 forwards, plus a 4-player bench (one per position). The bench is not decoration. Bench players score full points and count against your salary cap exactly the same as the players on your field.
Beyond the 23, you can hold unlimited reserves. Reserves never score... they're your storage shelf for cards you own but aren't fielding this round.
The salary cap
Every competition has a cap, and a card's cap cost depends on its rarity, not on how good the player is. Bronze cards cost $500k of cap, then it steps up per tier to $1M for a Legend. Field and bench both count.
That design means cap management is really rarity management: the question is where your expensive multipliers do the most damage, not whether you can afford a star.
Where other sports differ
NRL runs 18 players and counts your best 17. Cricket counts your top 11 scorers. Football (the round-ball kind) scores by position, so a clean sheet pays a defender and does nothing for a forward. The app shows the right shape for each sport when you build; the full-squad rule and the cap logic are built into the game so you cant get it wrong.