About 30 I would say
Chat with our AI personalities
If you are good at catching legendaries, then you would need at least 10 but 15 would be safe. If you are not good at catching legendaries, then I would get 25 ultra balls.
Take at least 18
Usually ultra balls are sold in the later cities try blackthorn city or the Pokemon league.
Beat her more times or capture Giratina on the Spring Path in the back of Turnback Cave. Or go find Buck and then go to the end of Stark Mountain to capture Heatran. For Giratina get Ultra Balls and for Heatran get Dusk Balls or Ultra Balls. Have FUN!!
i would suggest dusk and ultra balls but if you want to capture it without lowering its hp get a friends lugia and use repeat balls