Clarification on 2 child Tax Credit limit

I've tried searching for the answer to this but it doesn't seem to be clear. Unless I'm not searching well enough.

We get tax credits for three children currently, two of whom are leaving education this summer, so will no longer be on the claim. The third is a baby and will be the only one left on the claim come September. If we were to have another baby in a year or so (it will definitely be after September now, as dates would go), would we get tax credits for two children, or just the existing baby? As we'd be under the two child rule at that time, but will have previously claimed for more.