The Wikipedia link about the accusative case explains that
Modern English, which almost entirely lacks declension in its nouns,
does not have an explicitly marked accusative case even in the
pronouns. Such forms as whom, them, and her derive rather from the old
Germanic dative forms, of which the -m and -r endings are
characteristic.
Now, whether to use who or whom in your sentence entirely depends on which case should be used, accusative (whom) or nominative (who).
In English, it is grammatically correct to use nominative after the verb to be as in
It's he who stole my car. It's they who told me the truth. It's she
who lied to me.
However, we know that "It's me" (using the accusative case after to be) is broadly used in English. But it is just a few exceptions.
In your sentence, it is appropriate to use the nominative case as it is the complement of to be. If you divide the sentence into two parts:
He must decide / He should be who => He must ask who he should be => He must decide who to be.
in the same way as:
He must decide / He should meet whom => He must decide whom he should meet => He must decide whom to meet.
He must ask / She is who => He must ask who she is. (This question cannot be shortened with wh-word + to-infinitive as the subjects are not same.)
We don't ask,
*Whom is he? or *Who is him?
*Whom am I? or *Who am me?
because whom and him/me are the accusative case and can't be a complement of the verb be in this case.
Note: "He must decide who he wants to be" is more idiomatic than "he must decide who to be".
"It's me/I they want!"
You may be confused because of the existence of sentences like the following:
"I am [whom they want]."
Here, the subject is "I." The complement is the relative phrase "whom they want." The relative pronoun "whom" is prescribed in this context because it is the object of the relative clause.
The sentence you're asking about has a different structure:
"It's me/I [they want]!"
This sentence is definitely not equivalent in structure to "They want me," so just because "me" is the object of that sentence doesn't mean it's an object in your sentence. I actually don't remember if "it" or "me/I" is considered the subject in sentences like this (I'd guess "it" because the verb agrees with it), but it doesn't make a difference for your question because the main verb is "to be". Prescriptively, the complement of the verb "to be" is supposed to have the same case as the subject, which in this case would be the nominative.
Unlike "whom," "I" and "me" are personal pronouns, not relative pronouns, so it's not possible for "me/I they want" to constitute a clause.
Since "me/I is not part of the embedded clause, the case of the pronoun is just determined by its role in the matrix clause (similar to your example "It is I whom they want" or the situations discussed in the following questions: Which is grammatically correct: "Let he who..." or "Let him who...", Is it acceptable to start an emphatic sentence with "It is he who…"?).
So the prescribed form would be “It's I they want!”
But "It's me they want" sounds grammatical to me, and I'm pretty sure to many other English speakers, so that's also OK (see Which one is correct to say: "It's me" or "It's I"?).
Interestingly, though, "It's I" is still more common than "It's me" when followed by a relative clause starting with the subject pronoun "who," according to Barrie England's answer to the following question: It is I who am at fault? Possibly, the reason is because many people find sentences with first-person verb agreement but not "I" (such as "Remember me, who am your friend”) jarring.
“Who is it you asked?”
This is just the question corresponding to the statement "It is __ you asked." When you form a question, the prescriptive rule is that the interrogative pronoun takes the same case as the corresponding word in the equivalent statement. (In normal speech in real life, the situation is more complicated since we can say "It's me" but not "Whom is it?")
Since we established that the pronoun in "It is me/I you asked" is not a grammatical object, we would use "who" and not "whom."
Best Answer
In 'many of whom', 'whom' is the object of a preposition (the preposition being 'of'). As an object, it takes objective case. In formal writing, 'whom' is used as the object form, but informally, many people use 'who' for both subject and object.
Some other pronouns that have two different forms, a subject form and an object form, are
he/him
she/he
they/them
If someone isn't sure if 'who' is an object or a subject in a particular sentence, they can often work it out by substituting a different pronoun they are more familiar with.