Learn Italian: Ci sono molti musei in centro. - There are many museums downtown.

Ci sono means "there are" and is used before plural nouns. Because we are talking about multiple museums (musei), we must use the plural form. If there were only one museum downtown, you would use c'è (there is).

I remember molto meaning "very" or "a lot". Why does it change to molti here?

When used right before a noun to mean "many" or "a lot of", molto acts like an adjective and has to match the noun's gender and number. Since musei is a masculine plural noun, the ending changes to match, giving us molti.

Is musei the regular plural of museo?

Yes! Most masculine nouns ending in -o in the singular change their ending to an -i in the plural. This makes un museo (a museum) become musei (museums).

Why do we say in centro without "the", instead of nel centro (in the center)?

In centro is a common fixed phrase in Italian that simply means "downtown" or "in the city center." For certain everyday places like downtown, the bank (in banca), or the pharmacy (in farmacia), Italian uses the preposition in all by itself, without combining it with a word for "the".