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.
i think for starters they should not follow the rule of what they can afford to lose, they should start investing below that especially for those who clearly doesn't have any background or idea on what is bitcoin trading, well if they can find someone who could teach them the basics and the technicality they can invest whatever amount they want as long as they trust that person.