Is there any function for...

[GamerDeMTA]

Get the number of players in-game right now? in the server? Also say how to put in in an outpuChatBox please.

[Mega9]

getPlayerCount 

And a Wiki example:

function playerCount ( ) 
    outputChatBox ( "There are now " .. getPlayerCount() .. " players on this server!" ) 
end 
addEventHandler ( "onPlayerJoin", getRootElement(), playerCount ) 
addEventHandler ( "onPlayerQuit", getRootElement(), playerCount )