Hi, Mr. Hussein,
I'm glad to see you, too. Yes, I've read Gustavo's reply before. Actually, I have said I completely agree with him. I know that most native speakers would use his examples, but my idea is that since 'number' can be use as an uncountable noun, so why not use 'less' before it? I said that according to what we teach, I see your first sentence is 'correct'. I don't know why your supervisor have added 'a' before 'less'. There is no need to add it.