It all depend upon the investor's background of financial status as everyone has their own strategy where no one can take decision on behalf of them so it is not so easy to advise what is the good amount of bitcoin to buy. Any one can buy any amount and can fix their target of returns which they want to achieve accordingly they can invest in bitcoin.
But for the start, it is always advised to take first step in trading or investing with little amount. Only that much amount should be put in game which you can easily afford to loss. Because that would be your first experience and all your learnings are going to be practical so be mature to take better steps. And best step among all others is to choose little capital rather huge amounts.