Most Helpful Girl
It's not wrong to think about the question in general... it's natural curiosity. Some partners won't care and just answer you straight up. Others might worry about how your impression of them might change if they say a number too many or too few, so they might get flustered/defensive. If you want to know, you could always ask though.
But tbh if she's not complaining about the number of girls you've been with, and she's still with you, then you can't really act on your complaint of feeling gross thinking about how many people she's been with, either.