How many hard questions one can encounter?

The ESR after Verbal 44 and Quant 47 showed the difficulty level as "Medium-high" throughout both verbal and quant.

The ESR of a 800-scorer showed the same.

Point is, beyond a certain level all the questions are "hard". There are degrees of hardness of course, and some questions are harder than others.
But sometimes 'harder' just means that it's easier to goof up and very few test-takers get the question right.

What is hard for an individual is personal. The GMAC decides difficulty level based on statistics on how test-takers perform.

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To a large degree, the answer to your question depends on what you consider "hard."

If we go with the assumption that you are rather skilled in GMAT quant and verbal and thus that only questions that are among the hardest are hard to you, then we can say the following.

In quant, there will be around 6 to 8 hard questions, of which maybe 2 or 3 will be particularly challenging.

You could score 700+ by getting just a few of those correct, and if your verbal score is in the 40s, you could score 700+ without getting any of those questions correct, because you can score 47 in quant without getting any of those correct.

In verbal, there could be around 9 to 11 hard questions, of which around 3 to 6 would be particularly challenging. To score in the 40s in verbal, you generally have to get all but 5, or possibly a few more, questions correct. So, you have to get some hard questions correct.

However, you could miss all the hardest questions in verbal and score 700+ if your quant score were high enough and you got all the medium verbal questions correct.

For a high level test taker (e.g. Q49-Q50 level), the numbers are clearly much higher than this, on most tests. I'm not sure where you've gotten these numbers from.