First of all, let me say that I am in complete agreement with Gustavo. I find all of Mr Hassan's examples unequivocably ungrammatical.
I would find his example "•" (as opposed to his other examples, "•", "•", and "•") acceptable if "less" were changed to "lesser". The word is not commonly used except in certain set phrases (eg, "the lesser of two evils"), but because of such phrases it is commonly understood. Mr Hassan, was there something in particular that made you not want to use index numbers or letters for your four examples? This omission makes it very frustrating for me to respond to this thread.
Ahmed_btm, you wrote:
I'd like to refer to the fact that in our curriculum, we have the following sentence:
The tour guide said that ten is the least number of tourists she can take on the boat trip.
This is technically correct, but I'm not particularly fond of this usage. The first time I heard it was in elementary school in reference to the mathematical term "the least common denominator", which I found confusing at the time. It sounded like it should mean "the most uncommon denominator". I've never heard the phrase since elementary school. People now say "the lowest common denominator", which is much less ambiguous. In your example, I would prefer "the smallest number of tourists".
In any case, none of Gustavo's or Mr Hassan's examples even mentions the word "least", and what you say cannot be applied to "less", "few", or "fewer".
I don't know why your supervisor have added 'a' before 'less'. There is no need to add it.
I agree. We don't use an article before "less" when used as an adjective (*"a less milk than before"), but we can use one when "less" is an adverb modifying an adjective ("a less difficult path"). Also, as I said before, we can use an article with the comparative adjective "lesser" ("a lesser number of people").
I don't believe it's possible to use "less number" in a proper sentence.