Text (5th+errata): */*. ; ~this~'s power and toughness are each equal to the number of creatures named Plague Rats on the battlefield. [Oracle 2009/07/08]
"Plague Rats in play" means Plague Rats in play controlled by any and all players. [D'Angelo 1994/04/01]
Only counts cards that are "Plague Rats" and not all cards that are "Creature - Rats" (or "Summon Rats" on older cards). [D'Angelo 1994/06/01]