Pack Rat's power and toughness are each equal to the number of Rats you control.
, Discard a card: Create a token that's a copy of Pack Rat.
10/1/2012 Pack Rat’s first ability counts any creature you control with the creature type Rat, not just Pack Rats.
10/1/2012 The token will copy Pack Rat’s two abilities. Its power and toughness will be equal to the number of Rats you control (not the number of Rats you controlled when the token entered the battlefield). It will also be able to create copies of itself.
10/1/2012 The token won’t copy counters on Pack Rat, nor will it copy other effects that have changed Pack Rat’s power, toughness, types, color, or so on. Normally, this means the token will simply be a Pack Rat. But if any copy effects have affected that Pack Rat, they’re taken into account.
10/1/2012 If Pack Rat leaves the battlefield before its activated ability resolves, the token will still enter the battlefield as a copy of Pack Rat, using Pack Rat’s copiable values from when it was last on the battlefield.