Here are the rules about how many copies you can have:
* You can have up to 3 copies of any card in your main deck. This applies to monsters, spells, and traps.
* You can have up to 1 copy of any card in your Extra Deck.
* There are some exceptions to this rule:
* Some cards are specifically limited to one copy per deck. These cards are noted with a "*" on their card text, and often have powerful effects.
* Some cards have a limit on the number of copies you can have in your deck due to official banlists. These lists are updated periodically and you can find them on the official Yu-Gi-Oh! website.
So, as long as the card is not on the banlist and doesn't have a "*," you can have up to 3 copies of it in your main deck. This allows you to increase your chances of drawing and using the card during a duel.