10ドルで何個買えますかって英語でなんて言うの？

To ask for as many items as you want for a specific amount of money you can ask:
How many can I get for 10 dollars?
How many - The number of items
Can I get (informal) - Can I purchase (formal)

The shortened form is:
How many for 10 dollars?

I hope that helps!

We use "How many" with a plural noun or without the noun and also for countable nouns.

"How many can I get for $10?"
We omit (don't use or leave out) the noun if the person knows what you are referring or talking about.

Example of not using the noun:: "I need some coins. ( here we use the noun)
"How many do you need?" (here we do not use the noun)

Well, you may ask a direct or an indirect question.

Easiest is just to ask directly:

"How many do I get for $10?"
However, it is slightly more polite and shows that you have no definite intention of buying when you enquire:
"How many do I get for $10?"
"You will get approximately 25 for $10."
"Thanks."

If you are unsure of how many/much of something you can get with the money you have then you could ask 'How many of these can I get with 10 dollars?'
When in a shop you can also use the term 'purchase' or 'buy'

When wanting to know how many of an item can be bought with $10, you can ask "How many/much can I buy with ten dollars?"