We're going with 18, which probably seems light, but when you consider we have 11 mana dorks (and 14 if you count Marwyn, the Nurturer) along with four copies of Once Upon a Time, the number of mana sources is actually much higher
Graded Population Report: Lugia Legend #29 Visualize the distrbution of grades in the chart
This can make it difficult to use them reactively in order to make opponents waste cards and mana while you protect your gameplan
Whenever a creature enchanted by an Aura with umbra armor would be destroyed, you instead remove all damage and destroy the Aura, so targeted destroy-based removal spells are no longer a 2-for-1
Experimental Confectioner Ok, I have no idea if this card is going to be any good, but GB Food needs some kind of win the game engine the way RB has Mayhem Devil
Finally, Destined Rivals also has a Dondozo ex that deals more damage proportional to the number of damage counters on it: because it has 280 HP, it can potentially deal up to 300 damage with a single attack