Quake3World

I know that by default Q3a uses port 27960.

I am looking to run multiple Q3a server on one machine how far apart should I space the ports?

What range of ports will the Q3a server browser pick up on a lan?

You can just enumerate the ports but only the server on port 27960 will be picked up by LAN clients (internet servers OTOH will normally report their existence to a master server). Clients can still connect directly to the other servers by entering the IP address and the port number in the Q3 multiplayer menu.