https://medium.com/the-making-of-whereby/what-kind-of-turn-s... fippo pulled the stats and saw 17%
This also depends on your clients network. You can determine your NAT mapping type[0] and now ahead of time if you need a TURN server.
To understand NAT behaviors more check out [1]
[0] https://github.com/pion/stun/tree/master/cmd/stun-nat-behavi...
[1] https://webrtcforthecurious.com/docs/03-connecting/#nat-mapp...