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.
If we accept the Star Wars Christmas Special as canon, we see that Lumpy is an only child. This indicates that litter sizes for Wookies are either small, or that there is an unusually large infant mortality rate for this species.
And if I were to commit the original sin of xenobiology, we could infer that Wookies are primates, possibly great apes, which are not known for large litters.
Finally, raising something as stupid as Lumpy for such a long time means that Wookie offspring require a lot of resources to reach maturity. This is bolstered by the notion that Wookies have a lifespan of 400 standard years, which may indicate a childhood of 20-50 years, further increasing the resources required to raise an offspring to maturity.
However, none of this removes the possibility that Lumpy ate his siblings.