How to say "no" to plans - please help!!

Hi guys - please help me. So I'm an introvert and I hardcore need time away from peeps. I could be on my own for days and be perfectly content.

With that said, people tend to really like me. (this sounds like a humble brag but you guys know what I mean.) I am constantly having different people ask me to do something - grab drinks, grab coffee, see a movie, go to a party, go for a walk, etc. Of course I am happy to do these things now and then, but I really have like 2-3 social event (per week) max before I am just NOT a happy camper and not feeling calm/peaceful in my own skin, if that makes sense.

So, my question is, how do you say no to plans when your only excuse really is "I have a max hang-out quota, and others already claimed the open slots for this week?" (since I would obvi sound like such a dick for phrasing it like that!)