Several times I’ve cast necrotic wounds with more than 12 cards in my graveyard and it didn’t give the create - 12/-12.
It gave some other amount.
I haven’t quite figured out based on what…
//Edited Title -Brigby
Its based on the number of creatures, not number of cards.
@Furks Did you have 12 creatures in your graveyard or was it 12 cards consisting of different card types? Like Mburn7 mentioned, Undergrowth only counts the creatures that are in your graveyard.
I misread the undergrowth mechanic. Its working correctly.