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

I realise there has been a lot of discussion of who vs whom on these forums, but as far as I can tell none of the previous posts answer my question. Which of these sentences is (more) correct, and why?

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

OR

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

Please assume that the writer isn't taking the stance that who should be used all time, but instead wishes to use whom where it would be traditionally appropriate to do so.

Answer

‘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)