Learn Danish: Der er fire stole i værelset. - There are four chairs in the room.

No, cardinal numbers from 2 to 10 stay exactly the same no matter what gender the noun has. Only the number 1 changes depending on whether the noun is an en-word or an et-word (en or et).

Why do we use er here if we are talking about multiple chairs?

In Danish, verbs never change based on whether the subject is singular or plural. The verb er translates to both "is" and "are" in English. Because of this, the phrase der er is used for both "there is" and "there are".

How does the plural work for stole?

The singular form is en stol (a chair). To make it plural, we just add an -e to the end, giving us stole (chairs). Adding -e or -er are the most common ways to make words plural in Danish.

How do we get værelset from the basic word for room?

The dictionary form is et værelse (a room). To say "the" room, Danish takes the -t from et and attaches it to the end of the noun. Since værelse already ends in an -e, we just add a -t to make værelset.