Relentless Rats is a mythic Creature — Rat card from Special Guests (SPG), collector number 57. Current market price: $8.26 (foil $10.89), TCGplayer market prices via Scryfall.
This creature gets +1/+1 for each other creature on the battlefield named Relentless Rats. A deck can have any number of cards named Relentless Rats.