I think limit for buying is your amount of money, you can non stop buying bitcoins till your money ends. I am buying 0.2btc every week cause i don't need more.
Yeah you need to buy what you can afford, as there is no minimum or maximum limit to buy bitcoin, you can buy according to your wish and capital that you hold.
I think now he should have to invest all of his wealth in bitcoinif he really want to get bitcoins and to earn profit from it with increasing value, otherwise he will toss his head to the wall that why he did not bought at that price.
You only invest what you can afford to lose do not invest all you have, there is no limitation on buying bitcoin you can buy how many as you want but look for the risk of it. Just take care