Learn English – Can “who” be used to refer more than one person, in the following sentence

The pronoun who can be used in singular or plural, but the example sentence as it stands is grammatically incorrect and semantically unclear. Perhaps the intended meaning is

It is said of those who pretend not to hear what you are telling them.

Edit: As noted in a comment, use preposition of if the example refers to something said about someone, and preposition for if it refers to something said for the benefit of someone or on behalf of someone. Regardless of which preposition is used, those who is needed for grammatical correctness.

Edit 2: Regarding the question

Shouldn't it be: It is said from those if it is referring to the origin of something?

It is said from those is unsound; to say from does not have a clear and usual meaning there. (However, say from could appear in a sentence like “One can't say from that evidence what happened”; except see from or tell from are more likely verb forms in this latter example, and in my vernacular I'd say “From that evidence, one can't tell what happened”. That is, in several of these forms, substitutions are likely to occur in ordinary speech, to reduce clumsiness of phrasing. )

Anyhow, to reiterate, say from doesn't work in the original example. If you want to refer to the origin of something, do so explicitly. For example:

From the example of those who pretend not to hear what you are telling them came the saying [whatever].

Regarding the question

Shouldn't it be It is said for those who when referring to someone?

It is said for those who is an appropriate form when something is being said on behalf of, or for the benefit of, or to influence, some group of persons. But when referring to someone, of or about are appropriate prepositions to use. For example: It is said about those who dither and It is said of those who dither have similar meaning and refer to people who dither. Or, if something is said directly to certain people: It is said to those who dither.