Should I use who or whom when the subject is plural? [duplicate]

Solution 1:

‘Who’ does not inflect for number: it is always ‘who’ as the subject of a clause and ‘whom’ in all other contexts, whether its antecedent is singular or plural.

That said, your phrase is rather ambiguously worded (have you only met Pelé, or have you met all three, or have you met a lot of South American footballers, including Maradona, Garrincha, and Pelé? Or have you met all the South American footballers that are coming to the party). It doesn’t flow very well, and there seems to be some kind of determiner missing at the very beginning: if you remove the (I presume) parenthetical clauses, you’re left with the sentence, “South American footballers are coming to the party”, which is not incorrect, but sounds like a newspaper heading where determiners are often left out. Adding ‘many’ or ‘a lot of’ would make it sound more natural. I would suggest:

Many South American footballers are coming to the party, including the likes of Maradona, Garrincha, and Pelé, all of whom I have met.

(Assuming of course that those three were the ones you intended to single out as having met)

Solution 2:

If you are taking the view that you want to use whom when it's correct to do so (and not simply default to who) then...

Whom does not inflect for number, just as who does not. Whom indicates that it refers to the object of the verb.

In your examples, whom is correct [as is who, but whom does refer to the object of met, so whom can be used].

However, it is slightly ambiguous because it could refer only to Pele or to all three footballers. A viable disambiguation might be

South American footballers, including the likes of Maradona, Garrincha, and Pelé — all of whom I have met — are coming to the party.

[Note that whom in the prepositional phrase all of whom cannot be replaced by who.]