It depends on so much. Whether they take 30 or 90 per year group. A bigger year group = more chance of someone leaving or turning down their place.
What the area is like? Do people tend to move around or is it an area where families stay put for years? If people move away for jobs or other schools, you've more chance of a place coming free.
It is also possible some people won't turn up in September because they've got a private school place or decide to Home Educate but just haven't told the council yet.
Good luck - I hope you get in