Oracle Text

Rat creatures have fear. (They can't be blocked except by artifact creatures and/or black creatures.)

T, Sacrifice a Rat: Create X 1/1 black Rat creature tokens, where X is the number of Rats you control.

Card Rulings

12/1/2004 Marrow-Gnawer’s ability counts the number of Rats on the battlefield when it resolves, so you don’t count the sacrificed Rat.