Why the question?
State school
DS1 in yr 1: 30 children per class + teaching asst there nearly all the time.
DS2 in yr 3: 30 children per class, no TA. But for lit and numeracy hour every day 10 children are taken out in two groups with support staff. So the teacher just has 15, iyswim.
In addition school has lots of extra support staff, so there is another 'floating' assistant in yr1&2 (4 classes), there is usually a student in every KS1 class all the time, there are other specialist teachers, so for all lessons in IT, dance, and music the teacher is 'assisting' whilst a specialist is teaching.
Our school is a big school, always oversubscribed, therefore full: 2 classes per yr, 30 children in the class. But as a result budgetting is fairly easy, and they can afford to employ lots of extra staff, which balances against the disadvantage of larger class sizes.