I guess this isn't a complete duplicate, but a similar question has an answer here:
Pokemon cards with the same name and different features
You can play up to 4 copies of cards with the same name. If they have different names you can play up to 4 copies of each of them in your deck.
Rules text on the rule of playing multiple copies of a card (quoted from a quote in this answer: https://boardgames.stackexchange.com/a/47062/24175):
Also, except for basic Energy, you can only have 4 cards with the same name in your deck. This means you can have only 4 cards called “Pignite” in your deck, even if they all have different attacks. Finally, your deck must have at least one Basic Pokémon in it.
If you are wondering what it means to have the same name (quoted from a quote in this answer: https://boardgames.stackexchange.com/a/47062/24175):
WHAT DOES AND DOESN’T COUNT FOR A POKÉMON’S NAME?
• Some Pokémon cards have extra information after their name, such as a Level or a symbol like GL. A Pokémon’s name changes how you can evolve or play certain cards. Level is not part of a Pokémon’s name:
◆ Gengar, Gengar LV. 43, Gengar LV. 44, and Gengar LV.X all have the same name
• Symbols at the end of a Pokémon’s name are part of a Pokémon’s name:
◆ Alakazam, Alakazam 四, and Alakazam ☆ all have different names from each other
◆ However, δ (Delta Species) is not part of a Pokémon’s name. Aerodactyl and
Aerodactyl δ (Delta Species) have the same name
• An owner or form name in a Pokémon’s name is part of the Pokémon’s name:
◆ Meowth, Alolan Meowth, and Rocket’s Meowth have different names
When you are building a deck, you can have only 4 copies of a card with the same name in it, except for basic Energy cards. If your deck has 1 each of Gengar, Gengar LV. 43, Gengar LV. 44, and Gengar LV.X, you can’t put any more cards named Gengar in your deck! However, you could have 4 Meowth, 4 Alolan Meowth, and 4 Rocket’s Meowth.