I don't remember there being a firm limit for my DD's daycare. But I do know that biters are removed if it cannot be resolved. My friend had it happen with one of her children and was able to place her in a different daycare situation (not clear if the child is now in a small home daycare, but that was something they considered).
My own DD bit someone and I was told it was seemingly unprovoked. At around 1 yr, they get bitey. We were not told the victim (nor was the victim's parents told who bit him) just that here's a copy of the report, please initial that you received a copy and discussed it with the teacher. To that point, DD had only bitten me (for my attention, on my leg, and never deep) and we were teaching her that was Not Good. She did not get my attention when she did that. And, boy, was she quick! We worked hard with her to use words or signs and when she got riled up, she needed to remember not to bite. Playing "puppy" with a toddler can backfire sometimes.
DD was in a class with 5-6 toddlers and 2 teachers, so I wasn't worried about the supervision. Frankly, my own DD was bitten at a family event surrounded by adults. She and another toddler were playing off to the side at a baby shower. Then there was screaming. Initially I blamed DD since she bit someone before in daycare, but my cousin said no, it was the other girl. DD wasn't doing anything to the child that Cousin noticed, though it may have been conflict over a toy. And it was a NASTY bite, too. Almost broke the skin.
So I've been on both sides. I think the biter should have a chance to work it out but not to the point where other kids are going home every day with big bites. There has to be a line and maybe they've decided that 3 strikes makes sense.