Can anyone recommend a CRM that could help manage a monthly subscription service?

I've been having a tough time finding a CRM (or any software) that could help manage clients through a marketing agency who go on monthly retainers. Right now, I'm envisioning offering blog writing (and other things) on monthly retainers – so a client might get 2 posts a month, 4 posts a month, etc.

I'm looking for a solution that would make it easy to track a process like this.

I've looked into a ton of CRM options and just don't find anything that meets this specific criteria.

Any ideas?

EDIT: To clarify, I'm specifically needing a solution that would allow me to track client submissions for posts (they'd be filling out a standardized form for each blog post) and ensure they are not exceeding their monthly quota. I'd need the last part to be easy to review and report on. One example solution I've thought of would be, being able to issue clients "credits" every month that deplete as they submit their blog post form, then top up when the next month subscription payment gets issued. Just can't find a platform that would enable this.