A number is Khas because it fixes one quantity.
• “3 books” fixes the quantity at 3.
• “50 notebooks” fixes the quantity at 50.
• “This book was sold for 500 rupees” fixes the stated price at 500.
A number may describe many objects, but the quantity itself is defined and limited. That is why a fixed number belongs to Khas rather than Aam.
The sale sentence is a teaching example for fixed wording. It does not give a new ruling about sales.