This information can indeed be quite useful, and we presented a similar argument in the first part of our oversubscription appeal several years ago. You can get the figures from the annual census returns here: https://schoolsweb.buckscc.gov.uk/schoo ... n_roll.asp
Edited to add: the school will probably respond by saying that they had to take the extra numbers because of successful appeals. We pointed out that the GCSE results for a particularly crowded year 11 didn't seem to have suffered. This kind of argument is useful in "chipping away" at the council's case so that the panel is more inclined to allow at least some of the cases in stage 2 if it thinks the year group can cope with more pupils - however do remember that this point is just as likely to benefit another appellant's case as your own!