Nombre=Number Nombres=Numbers

Nombreux=Numerous (masculine) Nombreuse=Numerous (feminine singular) Nombreuses=Numerous (feminine plural)

If it's logical, do you have an example for such a plural form in French?

"Ce sont" (plural version of "c'est") is used if it is followed by a plural noun ("ce sont des oiseaux"); "ils/elles sont" if there isn't any noun following, just an adjective (like in this exercise).

Sometimes even if there is no noun, we do use "c'est", mostly in an exclamation: "c'est beau !" (instead of "il est beau !")

It means the same thing conceptually but is not a direct translation, which is what you are supposed to do for these exercises.

Agree

There is not ils. Ils is only they. There will be c'est. hope i helped

Because several is (much) less than numerous.

It's not a direct translation which is what you're supposed to do.