It only works once each turn, so you'll want to stack a large number of counters at once, or find ways to put counters on creatures during opponents' turns to trigger multiple times per turn cycle
Most decks in our Standard format have some way to accelerate Energy, like Archeops in Lugia VSTAR decks or Mirage Gate [Lost Origin] in Lost Zone Box decks, which allow Mew ex to shine in almost all viable decks
Partner Whenever you cast a spell with power, toughness, or loyalty equal to the chosen number, target artifact or creature becomes an artifact creature with base power and toughness X/X until end of turn, where X is the chosen number
Ok N [Reverse Holo] #105 (Pokemon Fates Collide) Details All prices are the current market price
PSAs grading standards are generally seen as more forgiving than companies like BGS, especially when it comes to centering and tiny surface imperfections