The first question is easy; use:
How many apples should you buy?
In place of should, you could also use would, could, did, will, or even might, depending on what you were trying to say.
In the second question, you can say:
How many apples do you buy?
However, by itself, I'd interpret that to mean:
How many apples do you buy on a regular basis?
I might fit that into a conversation like this one:
I go to the orchard every Saturday, and buy apples.
Really? How many apples do you buy?
However, if someone told me something different, I would ask my question in a different way:
I will go to the orchard on Saturday, and buy apples.
Really? How many apples will you buy?