So as in title, Is it possible to buy really large BTC amounts? like a million $? What was the largest amount of BTC you bought at once?
why do you want to know? are you planning on buying a million dollar worth of bitcoin
anyways it is possible if you have the money and can find someone willing to sell you that much bitcoin. it definitely is not possible through regular ways like buying it on exchanges.
there are also auctioned bitcoins that government has seized, they are in one batch and you have to buy it all or nothing and it is in great sizes.
but usually when you want to buy this much bitcoin you would do it slowly and with smaller amounts over a period of time preferably in the dips.