To effectively communicate the number of pages in a booklet for printing count every page front and back, starting with the front cover. This would be the same as if each page had a page number on it.
When printing, the pages will be printed on larger sheets, but it is most common to count the number of pages not the number of sheets.
For example using a sheet of 11x17 printed duplex (double sided) folded to a 8.5x11 booklet the number of pages will be 4. The first is the front cover, the second is the page on the left when open, third is the one on the right and fourth is the back cover.
The number of pages should always be a multiple of 4, if you would like the booklet to be saddle stitched, as a single sheet printed front and back will turn into 4 pages.
It is also important to count blank pages when communicating the number of pages in a booklet, because even if they are blank they will need to be accounted for in the layout of the booklet.
Using this standard counting method will help get your booklet printing quoted accurately and will also help with proofing, as we can refer to specific page numbers if there are issues in the design.