From the 2012 School Admissions Code
:"Academies, by which we mean Academy Schools, (including those that are Free Schools), University Technical Colleges and Studio Schools, are state-funded, non fee-paying independent schools set up under a Funding Agreement between the Secretary of State and the proprietor of an Academy (most commonly, and hereafter, referred to as an Academy Trust). Academies are required by their funding agreements to comply with the Code and the law relating to admissions, though the Secretary of State has the power to vary this requirement where there is demonstrable need."
and"Admission authorities must set (‘determine’) admission arrangements annually. Where changes are proposed to admission arrangements, the admission authority must first publicly consult on those arrangements. If no changes are made to admission arrangements, they must be consulted on at least every 7 years. Consultation must be for a minimum of 8 weeks and must take place between 1 November and 1 March of the year before those arrangements are to apply. For example: for arrangements which are to apply to applications in 2012 (entry in September 2013), consultation must be completed by 1 March 2012. This consultation period allows parents, other schools, religious authorities and the local community to raise any concerns about proposed admission arrangements."
So unless there are exceptional circumstances (a change in physical capacity due to fire might come into this category for example) they can't just change the number without consulting on it.