Currently I want to send a global message every 1 hour indicating how many connected players are on the server. How can I call the number of connected players or available slots on the server?

That a look at plugins like Player List or similar that do that.

Wulf

That a look at plugins like Player List or similar that do that.

Is there any way to also know how many slots the server has and if there are players in the queue in the same message?

Yes, both of which you can find by viewing the game's DLLs in a decompiler such as JustDecompile. I don't have an example on hand. There are likely threads that answer it already though.