I've reached 2000 simultaneous calls on a Dell Blade showing 40 CPU cores in /proc/cpuinfo , transcoding opus@8000h <-> PCMA with FS. Two years ago.
If you want to encode FEC you'll need to use opus@8000h as codec_string or tweak the config settings in the xml to set the encoder in Narrowband audio .
opus@16000h (newly added) should work with encoding FEC too.
When you prepare your setup use CLI command opus_debug and netem to introduce packet loss, this way you'll be sure if the phones are sending FEC or not.
Some may try to vary the bitrate to a value where the opus encoder simply does not encode FEC. Freeswitch, just like WebRTC stack sets by default the encoder in Fullband , which is CELT mode, and there's no FEC there.
It's all in the doc Giacomo shown.
Cheers,