As much as you love your photos, making a selection is a tiresome process. If you start with several thousand pictures from a long trip or even a whole year, it can take a while to go through them all, and select the best ones to include in your photobook. For many people, this is the least enjoyable part of the process.
Yet, selecting just enough photos for your book is the single biggest predictor of the time it will take you to finish the book. Choose too many photos, and you are now wasting time on every step: it takes you more time to select the photos, more time to upload them, and most of all, more time to curate each page.
Just imagine starting with 20 photos on each page: it will take you forever to pare them down to a reasonable number. You will spend time comparing them to each other, looking for the right layout, etc. etc. I personally have made that mistake often, and regretted it every single time. Believe me on this: delaying decision making when you are selecting your photos always costs you in the end. You will spend way too much time on each page, and you will find the whole process daunting.
A good rule of thumb is to assume four photos per page. For instance, if you are making a 50-page photo book, you should aim to select about 200 pictures in your first pass. Also, because you’re more likely to have a few full-page photos than to have 8 photos in a single page, you usually won’t use all of your first-pass selection in the final photobook: you end up with just enough extra photos to fine-tune your selection as the project takes shape in your mind.
I find that making a selection of pictures is like decluttering my drawers: I never end up missing any of the pictures I didn’t include in the first place. What’s more, if you end up deciding that you need more photos of a given theme (a particular event during a trip, or a particular person during an event), you can always go back and include them. So, do yourself a favor and be ruthless with your initial selection. The whole process will be much faster, smoother and more enjoyable!
As for the logistical aspect of how to make the selection: I personally use a very unsophisticated method of copying and pasting my favorite pictures from my general Photos folder to a dedicated Photobook Selection folder. You can of course use a specialized software, but I find that having my whole selection in a separate folder makes it much easier to then upload everything, and even reference it in the future, if needed.