The short answer?… You only “need“ one!
Yes, we live in a shaving environment that promotes faster, smoother shaves with more blades. But, the average man can get away with 1… maybe 2 blades. As long as effort is placed on how to properly prep and shave the beard (See my: Quick & Classic Shaving Tips), a single blade can be just as good as a shave with 3+ blades.
Of course, there are exceptions to the rule that may require the need for 3 or more blades (ie. a tough beard or a “twice-a-day” shaver). However, taking the time to customize your tools (creams, pre-shave oils, a fresh blade…) is the best way to approach any shave.
Hey, even I choose which blade is best to use at the shop depending on the beard!
So the bottom line is… Try not to rack your brain on the number of blades to buy. Start with the least number (See Post: A single blade that gets it close!), and work your way up. This way, with proper preparation and smarter shaving techniques, you will have your personalized answer in no time!