Hello
That is one of the nobody can really answer that type of questions. It depends... It depends many things broadcast levels, are they using multicast, bonjour etc.. In a perfect world it is a single /24 vlan but that hardly ever is enough. If you expand it to get more users and start to have issues we may need to do some policies on the Ethernet ports the AP's plug into to cut down/filter out any unneeded packets such as maybe all IPv6 packets or network neighborhood stuff. I know that wasn't a clear exact answer but it is the truth. Smaller the better, hope it helps.