"a stallion, two stallions"
Translation:un armăsar, doi armăsari
Armăsar is a masculine noun, so you have to use the masculine forms of one and two with it: un armăsar, doi armăsari.
I will give you two more examples of a feminine and a neuter noun, to point out the difference:
carte (= book; feminine noun): o carte, două cărţi
creion (= pencil; neuter noun): un creion, două creioane
As you can see, the neuter noun (as any neuter noun) behaves like a masculine noun in the singular and like a feminine noun in the plural. So unlike, say, some Slavic languages, where the neuter gender has its own identity, in Romanian it is more of a mix between masculine and feminine.