We only added the Serves field relatively recently so we didn't show that info on the site until we had enough data to make it useful. I think that is probably the case now so we will add that to the site soon. Obviously Serves will only appear where that data has been added when the recipe was indexed. I would be interested to hear from members whether it is important that they see the Serves info on the Bookshelf or Library recipe page or whether the Recipe details page (accessed by clicking the recipe title) would be sufficient.
As an aside, if Serves is useful for a book you own, you can request to add that data (as well as page numbers, if missing, and store-cupboard ingredients) by emailing us and we will assign it to you for proofreading. Several members are doing this - working through their cookbooks so they will end up with complete data for all the books they own.