Hello from MrBillDoesMath!
Answer: 270 stamps are in the set
Discussion:
If the number of stamps purchased is a multiple of 2, 3, 5, 6, and 9 then the count of stamps purchased must be the product of those numbers EXCEPT the number 6 is not involved . (6 = 2 *3 and both 2 and 3 already appear in the list of provided numbers. So their product, 6, is superfluous to this problem)
The number of stamps purchased is 2 * 3 * 5 * 9 = 270
Thank you,
MrB