A horse, a horse, my kingdom for a horse. Shakespeare.
An or a can only be joined with a singular: the correspondent plural is the noun without an article, as, I want a pen, I want pens; or with the pronominal adjective some, as, I want some pens.
THE.
The has a particular and definite signification.
The fruit
Of that forbidden tree, whose mortal taste
Brought death into the world. Milton.
That is, that particular fruit, and this world in which we live. So, He giveth fodder for the cattle, and green herbs for the use of man; that is, for those beings that are cattle, and his use that is man.
The is used in both numbers.
I am as free as Nature first made man,