Once every game (without cheats).
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you need to have the red orb and you need to have beaten the 8th gym leader, THEN you can have your very own groudon.
There is no Groudon in Pokémon Crystal.
Actually you will not be able to find a Groudon in pokemon black or white. It's because you will not be able to find a Groudon in the wild. The only way you can get Groudon in pokemon black and white you will have to trade with someone who has a Groudon or you can use Global trade.
Groudon is unavailable in heartgold. You can only catch Kyogre. If you want groudon, you will have to trade from heartgold, or migrate from another game.
Wailord is the largest Pokemon followed by groudon.