I mean, if you really want to know nipple amount you could see how many kids wookies normally have at a time.
He's gonna have pecs because he's bipedal tho. Those are arm muscles basically.
If wookies have a litter, then the females would have rows of teats, but they'd only have pecs with the top two. And they'd be basically invisible on a male.
So, we know Chewy has pecs, it's the existence of teats that is unknown
The muscle exists, but doesn't develop in quadrupeds because it doesn't get used.
An incredibly strong biped like a wookie should have massive pecs. Gorillas don't but that's because they're mostly quadapeds. Which I assume is what Lucas copied, but he changed the location without thinking how it would change anatomy.