I recommend having at least 3. If you look at a paticular bullet of let's say 150grs, and then look at all the powder recommendations for that paticular bullet in all 3 manuals, you'll probably see 3 different max loads for various powders. Not always, but enough that it might make a difference in your load. Let's say you've worked up to a max load in manual X, and you have no pressure signs but don't want to exceed a max load, but in manual Y, the max load is listed as .3grs higher or more. I have found this to be pretty common. It is always good to follow what ever manual you are using and to be safe. Just saying it gives you more information available to you to be better informed...