You can only select as many monsters with A-Counters as you have empty Monster Card Zones.
If a monster you control with the effect of "Mass Hypnosis" loses it’s A-Counter, then control is returned to the opponent.
If a monster you control with the effect of "Mass Hypnosis" is flipped face-down, then you will keep continue to control it, even though it no longer has A-Counters and even if "Mass Hypnosis" is removed from the field.