Each one holds six cards from a pool of "about 200" (the Pokmon website's being coy about the exact number for some reason)
It's a set of 64, including 4 extra deck sleeves, so it's enough for one 60-card standard deck!
Hydroid Krasis While Mulldrifter and Cloudblazer are staples in Blink decks, Hydroid Krasis may end up making a splash in Big Mana decks where you can consistently make X = A LOT
Second, Mirror March is a big dumb flashy 6cmc card that does nothing on its own, and Commander has a ton of those cards
Having Flash usually allows the cleric to (at worst) replace itself before getting annihilated by removal
If you can stare at a bunch of Alpha card corners/backs, and pick the questionable card out of the bunch every time for looking slightly off, then you may have cause for suspicion