How many f5 devices we can handel with 1 EM.

It seems there is no hardcoded limit. The perf of the system on which EM is running seems to be the only limit. Another article is about the same question (for EM VE), here

I'm not sure on the limit. We run a Virtual edition EM and have about 25 LTMs on it. Its a bit sluggish due to the resources the VM team gave us but it does back up all the systems and can do config searches. Stats collection is worthless because without an external data storage location for stats data base the onboard memory allocation for stats is only giving us about a day of stats. If we had set up an external data base for stats it would have worked better. If your using a VM for EM get as many CPUs and memory they will give you and set up an external data base for statistics collection and you should be able to support 25 to 50 devices. Right now we are looking and have BigIQ online but that has not been a very pleasant experience so far and it still doesn't have any Stats for trending so we are on hold until F5 can make BigIQ do something.