There's not a fixed amount of pokeballs needed to catch a pokemon. Though, there are many different strategies to catch pokemon, weakening it to red hp and putting it to sleep works best. (at least for me...)
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You need 21 ultra balls to catch Heatran
depends on how weak he is and how lucky you are
There is no specific number. you might catch on the first, or the 1,000,000,000,000th no one knows. it is just chance
Weaken him to red hp use thunder wave on him then use many ultra balls eventually you will get him.
You will get as many chances as you need to catch Cobalion provided you keep on defeating it in battle. Every time you beat the Champion, it will respawn all Legendary Pokémon that were defeated in battle that weren't caught.