Make a scoreboard objective to store the number of players:
/scoreboard objectives add count dummy
You can count how many players there are by doing this:
/execute as @a[tag=inGame] run scoreboard players add counter count 1
now the fake player "counter" will contain the total number of players with the tag "inGame"
now your command that will execute the other commands:
/execute if score counter count matches 1 run ...
and then reset:
/scoreboard players reset counter count
the counting part (command #2) and the reset part (command #4) should run in a loop and the testing command should be in the middle of them.
you know how to make a command block chain right? repeating > chain > chain, the thick one is your testing command.