Relentless Rats is a rare Creature — Rat card from Secret Lair Promo (SLP), collector number 11. Current market price: $6.05 (foil $16.14), 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.