Infant Class Size regulations state that there can be a maximum of 30 children for every one teacher. No more, unless they are an 'excepted pupil', which they can only be under certain very specific circumstances, and even then only for that one year in which they are admitted.
If a schools admission is 18 or 22, it might still be an Infant Class Size appeal, because there is no rule to say that they have to teach children only in classes of their own year group, and the ICS regulations apply to year R, 1 & 2.
So, they could have an admission of 18, but plan to teach 2 classes of 9 YR, 11 Y1 and 10 Y2, for example, meaning that each class would have a total of 30, so there wouldn't be any more space.
If they had an admission of 22 and only intended to have a class of 22, then yes, there would be scope for 8 parents to win on appeal before the next appeal became ICS. Having said that, those 8 parents would still need to show that the prejudice to the school by admitting, was outweighed by the prejudice against the child for not admitting.
Don't forget that PANs are not set arbitrarily. There is normally a very good reason why a school should only admit x children each year, such as space, resources, etc.