I was thinking about this just recently, can you use more monsters than what it says on the Xyz card? Like if one needs 2 monsters can you use 3 or more?
No, the card has a certain requirement of cards listed, and you fufill that requirement. No more, no less. xD Only time you can use as much as you want is is the card says "One or more -insert requirement here-"
Confirm what Eheroduelist said, you must fulfill the requirement written on the card and you cannot use more or less materials than the ones written on the card.