Learn Portuguese (Portugal): Há muitas nuvens no céu.

In Portuguese, the verb haver meaning "there is" or "there are" is invariable in this context. It always stays in the singular form há, even when talking about multiple things like muitas nuvens.

Is nuvens a feminine word? I noticed it says muitas instead of muitos.

Yes! The singular form is a nuvem (the cloud), which is feminine. Because the noun is feminine and plural, the word for "many" must match it, becoming muitas.

How do you spell the singular version of nuvens?

The singular is nuvem. In Portuguese, words that end in -m in the singular change the -m to -ns when they become plural. Other common examples include a viagem (the trip) becoming as viagens and o homem (the man) becoming os homens.

Why is it no céu instead of just em céu?

In Portuguese, the preposition em (in) automatically merges with the masculine singular definite article o (the) to form the contraction no (em + o = no). You need the article because Portuguese generally says "in the sky" rather than just "in sky".