An Ultimate Business Card book itself doesn’t need to be very long, but having a longer one isn’t a bad thing. Basically, for most trim sizes, your book needs to be at least 24 pages to get printed, and should be at least 60-80 pages to feel like a real book. In general, you can print up to 100-110 pages at the base rate before you have any additional per page costs, and if you get over 130 pages then you’ll be able to print on the spine of the book so that people can see the title when it’s on their bookshelf. So you can use those as general guidelines.
Note that these page counts are based on the final trim size, which will usually be 5×8 to 6×9 for a business type of book. Before formatting your book, that may only be 30-60 pages at 8.5×11 which is the standard size that a Word document might start at. You are generally looking at something between 10,000 and 30,000 words depending upon how detailed your book is.