For books with the inside pages printed on a lighter weight stock, in the 50-70lb range, the maximum page count would be 60 pages, plus the cover.
If a thicker paper is desired for the inside pages, such as 80 or 100lb, it is recommended to keep the page count below 36 inside pages, plus the cover.
The weight of the cover stock can also have an effect on the maximum page count, as a thick, heavy cover will reduce the number of inside pages that the stitching machine can accommodate. This is because the sheets are gathered on top of each other, stapled on the spine, and folded. If the spine is too thick, it will not fold nicely and can "bubble" open rather than laying flat.