cats

Between one and nine kittens will be born in a litter – most
commonly four to six. First-time queens usually have a small
litter size.

https://www.cats.org.uk/media/1029/eg18_pregnant_cats-_birth_and_care_of_young_kittens.pdf

combined with pretty fast birthing where she had contractions only for a few hours (my wife even went to work, fully expecting to have the five hours of time after the first contractions started about 4 hours before) and a pretty strainless birthing itself where she was still rubbing against my legs when i noticed the first kitten in its membrane starting to prod out of her) it's not dumb to expect that it wasn't her first litter at that point, but not before. i goaded her softly to her birthing box when i noticed that. It was an educated guess, not freaking science.