The 7210 controllers can terminate a maximum of 512 APs each. Although standard recommandations are not to max out each controller where possible and best practice also dictates not to terminate APs on your master (although it is OK in single controller deployments and sometimes done in failover scenarios).
The answer to your question really depends on whether you need a fully redundant environment. If so, 512 is the absolute maxium. If the master is serving APs simultaneously (not recommended) you could theoretically get up to 1024.