how to differentiate between Singular and plural (to a reader) when using a second person pronoun to ask a question?

Are you Rich ?

Could be asked to one person or a group of people.

is there a way to signal its singularity or plurality by itself.

a Solution For Ex: ( is you rich ? ) would definitely mean one , ( Are you rich ? ) would mean a group. provided it was grammatically correct

or

a completely different pronoun that represents different numbers ?

There used to be: the distinction didn't last beyond Early Modern English (around Shakespeare's time).

Art thou rich?
Are you rich?

Thou was the second person singular, and you was plural.

In modern standard English, there is no distinction and the number needs to be deduced from the context.

However, see also this question about dialects of English.