Learn Icelandic: Það eru þrettán stólar í herberginu. - There are thirteen chairs in the room.

No. Unlike the numbers 1 through 4, the numbers 11 to 20 do not change based on the gender or case of the noun. It will always be þrettán whether you are counting masculine words like stóll, or words of any other gender.

Why do we use eru instead of er after Það?

Það is acting like the English word 'there' in this sentence. Because we are saying 'there are' about a plural noun (stólar), the verb must also be plural. So we use eru (are) instead of er (is).

How do we get the plural form stólar?

The singular word for chair is the masculine noun stóll. To make it plural in the subject position (nominative case), you drop the extra l at the end and add the plural ending -ar to get stólar.

Why does the word herberginu have the ending -nu?

This is the neuter word herbergi (room) with the definite article attached to mean 'the room'. Because the preposition í describes a fixed location here, it forces the noun into the dative case. The dative definite form is herberginu.