Drills
Count enemies every ten seconds
In one line: For a whole match, check the minimap about every ten seconds and count how many enemies you know the location of.
Goal
Turn the minimap glance into an automatic gesture, and get used to knowing at every moment how many enemies you have located and how many you have not. It is the base of almost every mid-round decision: rotate, wait, go in.
How to do it
Play a normal match with a single goal. About every ten seconds, and always after a kill or a death, look at the minimap and count in your head: "five alive, two seen, A Main and mid". You do not need to tell the team: the drill is for you.
At first it will feel like you are losing focus on your crosshair. That is normal: after two or three matches the glance becomes short and stops getting in the way. If you forget for a whole round, start again next round without beating yourself up.
How you know it works
It works when, at any moment of the round, you can answer without looking: how many are alive and where you last saw them. A good sign is when you start telling the team "one is missing" before anyone asks.