There is no point system with ratios.
When working with mixed age groups the highest ratios are allocated first. Ie staff member 1 can have 3 babies, even if there is only 1 baby they can still only have 3 children, regardless of the age of the other children. If just one baby is being cared for, the entire ratio for that member of staff is 1:3. Same for the toddlers, any member of staff with a 2 year old within their numbers can only work at 1:4 ratios.
So from your reckoning they had 2 under 2s, 4 2-3s including your son and 5 over 3s.
Staff member 1 can be responsible for the babies and one toddler, staff member 2 can be responsible for the remaining 3 toddlers and one pre-schooler and that leaves 4 children out of ratio. And that's assuming they were correctly qualified.
I'd definitely say something. Ratios are legal requirements for a reason. Two members of staff is a tricky number anyway as once one adult is tied up the other person is completely on their own. Not great anyway but especially not when working over ratios and with such a range of ages and needs. And with 0-3s you're going to have a lot of changing and care taking a member of staff away so likely running with 1 adult to the remaining 10 children a lot of the time.
Also where were all these children? Is the nursery usually separated by age? Were they all in just one space and, if so, was that space suitable for babies up to preschoolers? Baby rooms look very different to pre-school rooms and I'd be questioning whether the space and resources were suitable for them all to share.