Asteroides's answer is spot-on, but since the comments express confusion with it, I'll try explaining it a different way.
Lac, as a mass noun, has no plural.
Much like you wouldn't say *"the boy has more milks" in English, you wouldn't say *plura lacta in Latin. (You could perhaps use the plural if you wanted to talk about a café with many different types of milk, but it still sounds strange to me.)
The problem is, the adjective plurēs/plura doesn't have any singular forms, since semantically it's always referring to a large amount of something. So you can't say *plus lac, with an adjective, either.
So Latin uses a different construction in this case: the neuter noun plus with a genitive of quantity. You can think of it as "a larger amount": the boy has "a larger amount of milk".
Canis, as a count noun, has a plural.
For canis, though, there's no reason to say "a larger amount": there's a plural canēs "dogs", so you can just modify this with an adjective to get plurēs canēs, "more dogs".
This is the standard way to talk about "more" of a count noun, though you can also use plus with a genitive if you really want to: plus hostium "a larger amount of enemies".