Answer:
3 books
Explanation:
Assuming that you cannot buy a partial book of stamps, you would have to buy a surplus in order to obtain (at least) 50 stamps. That is because 50 is not a multiple of 20.
In theory you would need 2.5 books of stamps, but that half-book should be counted and rounded up to three. You wouldn't have sufficient stamps if you buy only 2 books.