Depends on the state. My state is 1:3 for under 2, 1:6 for 2-3, and then 1:10 for 3-5. Presumably after that you're out of child care and into school. Ratios get more complex when it's a mixed group, but most childcare centers are going to have children separate based on age.
These ratios seem reasonable to me. Much better than the 1:25 in elementary school.