Diminishing Mage

Diminishing Mage enters the battlefield with a
return counter on it.
Whenever you cast a spell, if you've cast more
spells this turn than the number of return
counters on Diminishing Mage, put a return
counter on Diminishing Mage.
Your opponents can't cast more spells each turn
than the number of return counters on
Diminishing Mage.
Artwork source:
https://www.artstation.com/artwork/1BNeq