Numbers in Spanish are always singular. Plural forms of numbers don't exist. Look at the examples below. The noun is plural (houses, children, friends), but the number is not (five, ten, three).

cinco casas - five houses
diez niños - ten children
tres amigos - three friends

Feminine forms of numbers

Certain cardinal numbers in Spanish have feminine forms when used to modify feminine nouns. The number "one" has both a masculine and a feminine form: uno and una. For example:

sólo uno de tus amigos
- just one of your friends
Me gustaría saber sólo una cosa más.
- I'd like to know just one more thing.

Numbers ending with “one” may also have feminine forms, such as:
21 - veintiuna
31 - treinta y una
101 - ciento una

The hundreds forms may also have feminine forms, such as:
200 - doscientas
300 - trescientas
400 - cuatrocientas

Let's look at some examples:

Tenemos que saber doscientas ventiuna expresiones nuevas para el examen.
We have to know two hundred twenty one new expressions for the test.

¿Puedes prestarme quinientas pesetas hasta mañna?
Can you loan me five hundred pesetas until tomorrow?

Other numbers do not change for gender.

One cannot use “unas” for a number like 21, 31, etc. because the number is formed by addition:

treinta y una personas = treinta personas y una persona más