Un is the masculine word, there is a liaison with the following word starting with a vowel: j'ai un ami = j'ai un nami.
Une is the feminine form.
Deux, we don't pronounce the x, but in a liaison it is said like a z: deux amis = deu zami.
The same thing for trois= trois amis (zamis)
Quatre is sometimes pronounced kat.
Cinq is sometimes pronounced sin: sin livres.
Six and Dix have 3 ways of saying them, alone we say the final X like a S: sis, dis. Before a word starting with a consonnant, we just say: si or di: si livres. And before a vowel like a Z: si zamis.
We write Sept, but we say set!
Huit with a T but not before a cosonnant: hui livres
The F of neuf is sometimes like a V: neuf heures = neu veure but neuf amis = neu fami!
The T of Vingt is said in a liaison: vingt amis: vin tami.
Video lessons about liaisons with numbers in French: Lesson