Dying Population

Split Sceond(As long as this spell is on the stack, players can't cast other spells or activate abilities that aren't mana abilities.)
For each player, that player distributes X -1/-1 counters among creature they control and/or removes a loyalty counter from a planeswalker they control, where X is 20 minus the number of creature and/or planeswalkers on the battlefield. If X would be 0 or less, instead, put a -1/-1 counter on each creature and remove a loyalty counter from each planeswalker.