We have an HA freePBX installation, installed from the distro ISOs with over 6000 extensions on it. It is running fine. Each node is an HP Gen 8 with 32 cores and 64 Gigs RAM. The biggest hit with this many extensions is it makes the GUI painfully slow when it has to load all the extensions just to modify 1 in the GUI, but it all works if you are patient.
In order to eliminate voice artifact over 200 concurrent calls, we had to tune the kernel parameters as described in this post: Tuning freePBX Kernel Virtual Memory Parameters
Note that this system is primarily used for voicemail and IVRs, so our concurrent call level never goes much above 300. However, it also never uses more than about 30% of system resources. Your mileage may vary. 