Does anybody have any experience with this? I'm having trouble finding the number.
The only existing cap is the maximum integer size in java, which is 2^31-1 (2147483647).
The number is set from the server.properties file and can be as large as you like as long as the server has enough bandwidth and memory to hold all the people.
If you're asking about a particular server, the maximum number of connections can be seen from the server list in game.
There is no cap. 32-bit Java simply crashes after 2,147,483,647. If you had 64 bit Java you could probably go up to 9,223,372,036,854,775,807 but 32 bit clients that try to join your server might crash.
The Cap is 2147483647. this is a limit in place by java and not by mojang. If java had no limit, it would be infinite.
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.