If I am at 6-10 games, I can keep them all working "perfect". I usually have 1 project or restoration going it seems like which I don't count.
Right now I have 6 working at 100% as far as I can tell, and 1 with a playfield swap going on. This is in the zone for me.
When I get to about 15, I find that more than half of them have something wrong. So I am impressed with all the big collections with only a few out of action.