Learn French: Leur maison a douze grandes fenêtres. - Their house has twelve large windows.

No! Cardinal numbers in French, like douze (twelve), do not change to match the gender of the noun they describe. Whether you are counting feminine things like fenêtres or masculine things, the number stays exactly the same.

Why is it leur instead of leurs if multiple people live there and the house has twelve windows?

The word leur means "their", and its spelling must match the thing being owned, not the owners or what is inside. Because there is only one house (maison), we use the singular leur. If they owned multiple houses, it would be leurs maisons.

Why is the adjective grandes placed before fenêtres? Don't French adjectives usually go after the noun?

Yes, most French adjectives go after the noun. However, a small group of short, common adjectives describing Beauty, Age, Goodness, and Size (often remembered as the BAGS rule) go before the noun. Since grandes describes size, it comes before fenêtres.

Why does grandes have an "es" at the end?

French adjectives must match the noun they describe in gender (masculine or feminine) and number (singular or plural). Since fenêtres is feminine and plural, we add an "e" for feminine and an "s" for plural to the base adjective grand.