ESPDIC says:

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).

HTH