Melee can distinguish between a total of 352 control stick angles overall (source with image), with about a 17-degree range at the four orthogonal angles where it assumes you want pure horizontal or vertical. Each of these different angles will give you a different amount of DI. As DI is based on being orthogonal to the impending knockback, this means that in theory you have ~177 choices of DI. Obviously it isn't practical to try and pick one specifically, but it's how it is.
However, there is a wrinkle. While Fox's up throw is a perfect 90 degrees, Peach's is 88, and Marth's is 93. As a result, you don't get equal distance on either side: you can move farther sideways (but less vertically) if you DI Peach's forwards and Marth's backwards. In addition, because these angles are within 17 degrees of orthogonal, no "perfect maximum DI" angle for them exists (such angles get swallowed by the pure horizontal/vertical assumer).
In short: Treat DI as if it is analog and not digital, and realize that you get less DI out of attacks within 17 degrees of orthogonal (but not exactly orthogonal).