Check the competition rules - we have an OA league with roro subs, but the rules say they can name 5, but only use 4 - those four can can come on, and those they replace can come back on as often as they like, but there will be one Sub who can't. You don't know which one until they have made their first four changes (assuming a substituted player doesn't come back on before four named subs have been introduced).
Clear as mud eh?
Just make a note of numbers on & off.