(If you want to test without having to summon an entity, this applies to the FIRST set of commands Heklo posted and won't work with the second set!)

scoreboard players test playersOnline playersOnline <min number> <max number>

In most situations it's easier to use an entity like Heklo described (since you can test for it using target arguments instead of one specific command), but I decided to post this alternative just in case.

Summon an invulnerable pig called playersOnline and run these 2 on a chain in this order:



From here you could test for it by doing this:


If 5 players are online, it will result positive. Or you could use the following format to skip a testfor command and just spam [command] when 5 players are online:

Ok, nevermind still doesn't work. How do I then test for the value of playersOnline?

Ok, nevermind. I just forgot to put in a command into the 2nd block. It works now. Hopefully this part of the project will be finished quickly now.

Ok, thanks. Now one more thing: How to start a chain of chain command blocks with a button?

Run these 2 on a chain in this order:



The score of playersOnline in objective playersOnline will be equal to the number of players online.