Translation:Why do you want to have several pigs?
multaj : many, several; pluraj : more than one, multiple, several
I looked for some example sentences on the internet and came to this conclusion:
Pluraj means that there is "more than one" but not necessary "many/a lot". Multaj is used as "many/a lot".
FYI: Malmultaj would be "few". I have also seen malpluraj used as "few" but I am not sure if malpluraj is correct (definitely far less common than malmultaj so I would stick to malmultaj; google search: malmultaj 306 000 results; malpluraj 43 results).