Limit Paid Subscriptions

[Cable_Player]

Hello,
I've done a quick Google search but it only found a vB 3 MOD (https://vborg.vbsupport.ru/showthread.php?t=162476).

I'd like to limit the amount of times a paid subscription can be purchased and when one expires it becomes available again. Does anybody know of a modification which will allow me to limit the number of times a specific paid subscription can be purchased?

At the moment my intentions are to run a paid advert subscription and because I can only display one ad in a certain place at a time and not have them rotate I was thinking of doing it on a first come first served basis.

Thanks. (I'm using 4.2.1)