the /list command in a command block does not output a signal the size of the number of players online

I was trying to figure a way out to test for the number of players on the server and compare it to a dummy number. I would subtract the two and if everyone's ruReady dummy objective was set to 2 then doors would open. I was using "scoreboard players list" to get the number of players. However, that list outputs the number of people on the scoreboard and the scoreboard doesn't update when someone leaves the game. So for instance if I had 4 players and we all were ready (had ruReady set to 1) then the doors would open, but if that person left before they were ready then the doors would never open even though all the players online are ready because that one last person had not hit the button to be ready. So I thought about using /list instead of /scoreboard players list. List gives only the players online. However list only outputs one block of signal no matter how many people are on.