The page count will depend on many factors. It can vary even between different issues of the same catalog. The winter version can be longer than the other three seasons to include more products for Christmas.
Some things to consider when determining the best page count would be:
All of these factors are connected, and neither one is more important than the others. For example, a page size of 5.5x8.5 can fit either fewer products of the same size as a 8.5x11. Or you can have the same number of products, with smaller font size and images. Similarly, the smaller format would require more pages than the larger one if the design elements and the number of products per page are the same.
A catalog that accompanies a new collection/brand or a Look Book might have just one product per page. It can even have one product for each 2-page spread (two pages next to each other when the book is open). If there are 20 products, that would require at least 20 or 40 pages, plus the covers and other content pages. Alternatively, a catalog intended to be an exhaustive list of product offerings could have 4 or 5 products per page, or far more if it is a table-style list.
Finally, there are the non-product pages to consider. These are, of course, the front and back cover, but also other structural elements like a Table of Contents or Index. These pages must be included for the catalog to be useful to the reader. Most catalogs are saddle-stitched and require the page count to be in increments of 4. To determine the page count needed, you should first decide how many products will be on each page and in total. Certain products can be showcased, thus increasing the total page count. Then add the other structural pages to get to a preliminary "must-have" page count. These are the pages that cannot be left out. If this is not an increment of 4, extra pages can be added to bring the count up to one that will work for the bindery method.