The substantive form of alcuno, in other words the pronoun, matches in gender and number with the noun it refers to.
We have a similar problem as the one we had with the adjective forms. The meaning of the plural is clear, alcuni / alcune means some / any. But what is the meaning of the singular forms, alcuno / alcuna. Simplifying we can say that alcuno / alcuna as a pronoun (don't confuse that with the use as an adjective! Used as an adjective it can be used in singular) can' t be used in singular.
One might think that alcuno is used in negative sentences. That's not the case. In this case nessuno is compulsory.
I haven' t met anybody.
Non ho incontrato nessuno.
wrong: Non ho incontrato alcuno.
That may seem illogical, because the double negation (non ...nessuno) could be understood as positive, but that' s not the case in Italian (and Spanish). Double negation rests negation.
In some isolated cases, mostly in literature, you can find alcuno / alcuna as a postponed pronoun (don't confuse it with alcuno / alcuna used as an adjective => senza raggione alcuna / without any reason), but this use is very seldom.
And all that must happen without anybody coming to know it.
E tutto ciò deve avvenire senza che alcuno lo sappia.
The table below is a simplified summary of all we have said until know.