Can Malzahar have more than one voidling out at the same time?

Passive: On every fourth ability use, Malzahar summons a voidling for 21 seconds. There is a one second cooldown after summoning a voidling where further spell casts will not increment the counter.

Can Malzahar have more than one voidling out at the same time? If so, assuming he has max CDR how many can he have out for how long?