If it is uncountable stuff, like water or sugar: How MUCH of this did you buy? If there are 10 books in front of me, and I want to know if you bought 3 of them or all of them: How many of these did you buy? If there is a list of different types of objects (for instance, red bottles, black bottles, green bottles, etc) and I point to one item on the list (say, red bottles), I could either ask How many of this did you buy?=How many of this type of bottle did you buy? OR How much of this did you buy?=How much money's worth of this did you buy? OR How many of these did you buy?=How many bottles did you buy?

Many implies plural (more than one) ....so the second sentence is perfect..... If you change the first one to: How much of this did you buy? It would sound more natural, but it doesn't imply plural. So, if you were buying pencils or pens or some discrete quantity of items, the "how many of these did you buy?" would be best. If you were buying something buy weight in a grocery store, someone might say "how much of this did you buy?" Also, someone could say "how many did you buy" and that would be similar to "how many of these did you buy” the main difference being -of these-. With those words included, it implies that you are holding them or they are right next to you. Hope this helps!

How much of this did you buy? The second sentence is ok.