Hi Rahman,
so here what you'd do...
Controller:
The cheap solution would be the VM controller V2110 if you've a VM server already in place.
If you want a applicance the brand new C35 is the right hardware for you.
The V2110 could support up to 525AP (large VM server - please see datasheet for requirements).
The C35 could support up to 125AP.
Here the controller datasheet...
http://learn.extremenetworks.com/rs/extreme/images/Wireless-Controllers-DS.pdf
The thing is that your current C5210 has 4 physical ports - if you use all of them the best solution is the C35 as that one has also 4 interfaces.
The V2110 on the other hand has only 2 interfaces - so that could be a problem to map 4 interfaces from the C5210 to only 2 on the V2110.
You need to also check other things like max clients per controller to make the right decision.
i.e. C5210 mac clients is 16k but the C35 could "only" handle 4k.
Regarding the licenses - you don't need to buy AP licenses for the hot-standby controller.
-Ron