18.
Yea, that's the one. He's a slippery one to fight in Ash Gray. His moveset counters Ash's team at that stage. He has a rollout for pidgeotto and butterfree, and dig for Pikachu. Even if you pick up Bulbasaur early, the Sandshrew can rapid spin away the leech seed, and is packing fissure as a nasty surprise.