for a good staking how much coins must buy??

As many coins as you can afford.
Yes that is true, however if he wants to stake daily then I would suggest at least 50k to 100k RAIN. If weekly then the minimum amount should be at least 5k to 10k. The more the better but as some people can't afford much I think 5k is a good start and right now its a good time to buy while the price of RAIN is under 1k sats.