I wouldn't worry about how many are lost in one day. When Xander was a pup I found 5 on our floor over the course of just a couple of days. Each dog has its own maturing rate...and of course how much they like to use their teeth will also be a factor in how fast they might lose their puppy teeth. Just relax, sounds perfectly normal to me
I think dry food definitely affect dental and teeth hygiene of puppy.
Generally speaking, puppy food should higher in protein and enriched with vitamins, minerals, and fats essential for growth--that is, when compared to dog food made by the same manufacturer. Still, ingredients across brands can vary greatly.
Check this brand before going to any decision --------> PUPPY FOOD
It is ok for a puppy to loose that many teeth in a time. When this happened to one of my Yorkie, I could see his tiny teeth on the carpet, on the stuff toy he liked to chew and so on. The new set of teeth will come up so quickly so don't worry. Your foster pup is fine.