I would say 2 on cannon minions and 1 on melee minion. Mages don't need a Minion Dematerializer because you will one shot mages with full Q after second-ish back. Late game even melee minions will be one shot with Q so the priority is to maximise damage on cannons so you can splitpush really quickly late game. The 1 minion dematerializer for melee is to assist in early and mid-game shoving power.

You should look for opportunities to shove wave faster with Minion Dematerializer. If your enemy laner backs early-game, minion demat something from the wave (preferably cannons but can be melees) and hard shove quickly. Very helpful for instances like this to get free back or plate.