We say "the poor" but then we say "the Italians". If both are correct, I wonder why is not possible to say "the poors". Can you tell me the rule please?
Poor is an adjective.
Sometimes we use an adjective such as poor as if it were a noun. But when we do, we mean the poor as a class of people, meaning the poor ones. Even without the word ones, the poor is considered plural. So we do not add an s to it.
The ony way to make the poor singular is to put a singular noun after it: the poor boy. If you wanted to refer to more than one poor boy you could say poor boys.
Adjectives that work this way include poor, rich, meek, wealthy... Grammarly lists the following examples as adjectives that work this way:
aged, educated/uneducated, injured, powerful, unemployed, blind, elderly, living, rich, weak, brave, free, needy, sick, wealthy, dead, handicapped, old, starving, wounded, deaf, homeless, oppressed, strong, young, disabled, hungry, poor.
However, other adjectives used as nouns can mean a single person or thing: the former, the accused, the deceased. We have the option of putting ones after them and they will be plural: the former ones. But we still would not say the formers. In general, adjectives--even when used as a noun--cannot be pluralized.
So, yes, the dead is plural and is short for the dead ones and the deceased is short for the deceased one and is usually singular (but it can be plural: the deceased ones.). There is no rule, to my knowledge, for why this is so, other than how the language has developed over time.
Adjectives that refer to nationalities follow a different pattern.
Italian is used as both a noun and an adjective. In the Italian boy it is an adjective. In the Italian played a great game it is a noun. I think really the only way we know this is that you can pluralize it: The Italians played a great game. Not all names of nationalities work this way: it is rare to see the English used as a singular noun.