My 3 week old exclusively breastfeeding infant has 8-9 dirty diapers a day (many are smaller and many are also wet).. is that a normal amount? mom was on keflex for a week early on but recently stopped. could that have caused gut biome issues?

A breast fed infant can poop with every feed, often just smears, or a diaper filler once a week. By itself this is not alarming or a sign of any gut issue. As long as the kid is a good feeder & seems to be filling out/active I would not worry about it.

Congratulations on your new baby and on successfully establishing breastfeeding. Vaginally born babies are colonized with bacteria from their mothers. You are right in that it is best to minimize exposure to antibiotics, but your baby's microbiome should be fine.