For the maximum number of UDP clients exchanging with a server, please refer to those links I found:
Basically since UDP is connection-less, your server is only receiving and sending packets from clients and not maintaining connection states.
Quoting:
A UDP server could therefore send/receive traffic from a theoretically unlimited number of address/port combinations. The server code simply inspect incoming packets for source address and port, then sends any responses back that way.
So yes it will depend on your resources (network, hardware) and server code performance.