The only limit is on the swithces themselves, and they should be able to handle many more devices than you have listed.
Look at the switch's logs to see if there are port errors.
Look at SNMP data from the switches to see if there are any traffic hogs cholking any ports or any processor hogs messing up your switches.
I assume the two Cisco switching stacks are connected together by the fiber?
Check the fiber interfcaes for errors, drops in the links, etc. This could be indicative of dirty connections. You can clean the fiber patch cord ends with alcohol and lint free wipes. Make sure you wash your hands first to eliminate skin oils.
What is the distance of the fiber connection and what type of fiber are you using (i.e. multi-mode, single mode)?