Does anybody have any experience with this? I'm having trouble finding the number.
Wrong! You CAN SET the Slots to 2147483647, yes. BUT you have to connect to the Server and since there are Ports from 0 to 65535 there Could be 65535 Clients, then all incoming ports are full. But even that's not possible since Minecraft starts in mid of the portrange (idk where) so I think about 4000 Ports can be used. If you ask how big Server can have this much, the have multiple proxies and the Playercount is added together.