For Symphony Link, there are 8 hopped receive channels, each with 4 modulation rates. The gateway can receive 8 messages at a time, but there are many slots within a frame to ensure access. The limitations results when many devices are trying to transmit in the same frame. For 10 byte payloads, we would expect about 300-500 messages to be received in a frame. The other packets would be retried when the ACK failed. QOS suppression would also kick in if the gateway was "overloaded."
It's a hard question to give a straightforward answer to, because so many variables are in play.