The relevant portion of the creatures stat block:
Eat Memories. […]
While memory drained, the target must roll a d4 and subtract the number rolled from any ability check or attack roll it makes. Each time the target is memory drained beyond the first, the die size increases by one: the d4 becomes a d6, the d6 becomes a d8, and so on until the die becomes a d20, at which point the target becomes unconscious for 1 hour. The effect then ends.
The standard die for DND have 4, 6, 8, 10, 12, and 20 sides. Does the ability skip from 12 to 20? or does it does it continue to move up by 2 each time – hitting 14, 16, and 18 – as in the stat block? There is precedent for non-standard die size rolls, such as a d3 used in the Wild Magic Barbarian’s Bolstering Magic.