of client devices is IMHO irrelevant since the load will be shared across cap-devices (no capsman-forwarding in wifiwave2, only local forwarding).
Capsman is handled completely by CPU, so CRS might not be the best choice.
I have a warehouse setup with 15 caps handled by RB5009 running wifiwave2/capsman, also acting as DHCP server but not as WAN gateway (might be an important remark).
It's during work hours not even going over 10%. So the RB5009 is overkill for that setup