The advertised speed is up to 15 meters. But the actual speed I observe at my home is 2.5 meters for receiving and 1 meter for sending. For comparison, a regular 3G provides 1-1.2 meters for receiving and 300-400k for sending. It's enough for me, but I'm not sure what will happen if I connect 20 computers to it. There are no problems with playing games, but I'm not sure about streaming. At least, disconnecting a third or half of the devices from the main source will be beneficial.
And a Wi-Fi modem.

I didn't quite understand what you're planning to buy and why?...