Not sure if this has been asked before (I have searched for e.g. "skin limit", "livery limit", and "opponent limit" but didn't find anything obvious)... Currently the game will limit car numbers to the number of skins/liveries available for the car. Is there a reason for this, e.g. why can't liveries be repeated (or maybe hue-shifted) for larger numbers of cars/opponents?
They can be. The game will warn you about repeat liveries, but you can absolutely do it by simply setting opponent number to 'custom' and increasing the number. You can then even switch back to 'maximum' and it will always fill the grid up.
You can select number of opponents to custom and then increase the number of opponents to as many as the track will allow, I've done it before.