I have something like 15-18 soaps and creams.
I have no set policy on rotation. I rarely use the same soap on consecutive days, but may use it more than once in a given week. Why have so many soaps? Some drink to forget, some shave and forget. I have only been DE shaving for 10 months, so I still have a process of rediscovery going on. Soaps that disappointed are rediscovered and turn out to be better than I thought at the time. Some "winners" start to disappoint as my range of experience broadens. New brushes start to impact selection (boar brush for hard soaps, synth for creams).
There are a range of soaps I would have thrown out a few months back (Proraso, Palmolive stick), but having "got over myself" in terms of wanting to own edgier, more artisan soaps, I have found that cheapness and widespread availability are poor reasons to pass judgement. Conversely, there are some artisan soaps I have that are such high quality in terms of performance and scent that I would never let a week pass without visting them upon my face.
Here's the thing. If I was landed on a desert island with just my Gillette tech, supplies of Lea stick and Sputnik and a cheap brush, I know I could cultivate an excellent daily shave for the rest of my days. In fact, I'd probably get better and better shaves through familiarity. But none of those objects are my favourites in category.
What does this all mean? It means I select each day on the basis of whim. It's perhaps the only part of my day where I have complete, unfettered control over my choices. There's an interesting contradiction in play - shaving is a deeply repetitive single routine for each day in which I exercise control over something like 1,000 combinations of razor, blade, brush and soap/cream.