I have ~10,000 cards in a Chinese deck (learning characters in college ~20 years ago is how I started with spaced repetition) that I built throughout the 2000s when I in college and then living in Shanghai. I don't add much to it anymore, but it keeps my reading pretty sharp, even though I don't use it much day-to-day anymore.

Another ~10,000 cards are work-related (computer science). I add to this frequently, mostly consuming research papers and turning them into cards.

The last ~5000 cards are an assortment of topics I've been interested in over the years.

I don't think that I use it the way you're describing--I more or less know all of the things that come up in my reviews each day, though I certainly miss things from time to time, and sometimes I go through and purge a bunch of cards related to topics that I don't have any interest in. It's much less than my total capacity (I know plenty of things that I haven't encoded as flashcards). More than anything it's a way to artificially engage pathways I'm not actively using, in the hope that when I need them they'll be there. It feels much less costly to maintain knowledge than to forget and relearn it, though there's always the danger that you are wasting cycles maintaining things you'll never need again.