How many end-devices can a LoRaWAN gateway support?

End-devices do not actually make a connection with a gateway. End-devices broadcast to all gateways within range and the gateway forward packets to a network server if it can hear them. So, a gateway does not have a hard number for supporting devices and it is rather dependent on how much bandwidth a gateway can support. For example, factors such as payload size, interval of each data transmission or data rate being used can affect how much air time is consumed and therefore the gateway processes. Also environment factors should be considered.   

It's normally best to experiment with real devices in actual sites in order to figure out how many devices can transmit data without much data loss, using desired configuration/setup.