Learn Turkish: Biz yirmi arkadaşız. - We are twenty friends.

In Turkish, when you use a number before a noun to say how many there are, the noun stays in its singular form. You never use the plural suffix (-lar or -ler) after a number. So it is always yirmi arkadaş (literally 'twenty friend').

What does the -ız at the end of arkadaşız do?

It acts like the verb 'to be' for the pronoun biz (we). By attaching the personal suffix -ız to arkadaş, you are saying 'we are friends'.

Why does the suffix on arkadaş use the letter ı instead of i?

This is because of 4-way vowel harmony. The personal suffix for 'we' changes to match the last vowel of the word it attaches to. Since the last vowel in arkadaş is a, the suffix becomes -ız. If the word had ended in e or i, it would be -iz.

Can I leave out the word biz and just say Yirmi arkadaşız?

Yes, you can! Because the -ız suffix at the end of the sentence already clearly means 'we are', the word biz is optional. Turkish speakers often drop the pronoun unless they want to emphasize it.