No, a/an is only singular. It's only un or una, and never unos or unas. And it's not "uno".
Uno means "one", you use it when you count, and when there's no noun behind.
Because when you use a masculine noun with "uno" (and you are forced to do so with you use a noun, as "uno" is masculine), you will apocope it, it means that it loses its final "o", and become "un".
So, just remember that "un" is the masculine a/an,
but it some context (when counting something), it can also means "one".
Un café = a coffee (or one coffee in some contexts)
For "una" it's not apocoped: Una chica = a girl (or one girl in some contexts).
For "unos" and "unas", they are plural, and mean "some".
Unas chicas = some girls.
"Un" and "una", (unless you want to use "some"), don't really have plurals.
Most of the time, when you want to pluralize a sentence like:
Soy una mujer (singular) -> Somos mujeres (plural) :
You just omit the indefinite article (here: una) in the plural.
Like in English.
I am a girl -> We are girls
(and not we are some girls).
"Carro" is masculine, and needs a masculine article.
"Bicicleta" is feminine, and needs a feminine article.
There are exceptions, but usually word ending with "o" are masculine.
(but not all the masculine words are with "o", and not all the words with "o" are masculine)
And same for words in "a" being feminine (but there are exceptions, not all the feminine words... etc.. same than above).