Pages are charged based on the number of pieces of paper used, regardless of whether anything is printed on one side or both sides.
Therefore, depending on the number of printed sides and how many sheets of paper are used, double-sided printing may count as either 1 or 2 pages. For example:
4 pages printed single-sided = 4 page allocation credits used
4 pages printed double-sided = 2 page allocation credits used
5 pages printed double-sided = 3 page allocation credits used (3 sheets of paper are being used)
To see your print allocation use and how many pages you have left, log in to Web Print, and in upper left corner click on Summary.