This is the standard limit for any card in your main deck. There are exceptions for certain types of cards, like:
* Monsters: There is no limit to the number of monsters you can have in your main deck, as long as you don't exceed the overall card count.
* Spells/Traps: These also have a limit of 3 copies each, unless they are specifically stated as a "unique card" that you can only have 1 copy of.
Therefore, you can have 3 copies of Malevolent Nuzzler in your deck.