IMR4064 and Varget are nearly identical in burn rate, load density, and application. Data for the two is often 1 grain or less apart; sometimes identical for the same cartridge and bullet. I'm not saying they are the same powder (IMR is made by General Dynamics in Canada, Varget is made by ADI in Australia) or that they are interchangeable (they are sometimes though); just that having both when you're trying to limit the number of powders you keep on hand is kind of redundant.
Distilling your reloading down to one powder would require a collection only consisting of firearms that shoot similar cartridges (similar pressure curves and max pressures within a certain range; allowing them to use similar powders). Even going down to 2 powders for me would be practically impossible because of the large range of different cartridges I reload for.
Going to one powder for all reloading would be like going to one grade of gasoline for all your vehicles. If you only have vehicles that have similar compression ratios that can all use the same grade of gas, it all works. But if you have a car that takes premium, a truck that takes regular, a lawn mower that needs a 50:1 mix, and an older motorcycle that can't have any ethanol, it isn't going to work.
I'm not saying going down to 1 powder is a bad thing or it's a bad objective to try to have. I'm just saying it shouldn't be everyone's objective as it doesn't suit everyone.
I probably have 12-15 different powders on hand; mostly in small quantities for testing (1-3lbs of each).
The powders I try to keep a lot of on hand are Trail Boss, Titegroup, H110/W296, H4198, H4895, Varget, H4831, and Bluedot. Several of these can't be replaced by one of the others very well if at all. The first 4 are pretty much fixed as there isn't any suitable overlap between them. The H4895/Varget/H4831 could sorta be rolled into H4895 or Varget but my rifles that want slower powders will suffer (6.5x55mm, .300WM, and heavy bullets in 7.62x54R).
Bluedot being near impossible to find for several years has moved me to HS-6 and Longshot for slug and buck 12ga reloading.
I also keep Goex FFg and FFFg on hand but don't stock up because of the more limited shelf life of real black powder. Usually just 2-3lbs of each on hand (a 12-18 month supply).
So yeah, I could distill my 12-15 powders on hand down to 6 if I really had to in a pinch but ideally, closer to 8 powders (counting H110 and W296 as one since they're the same).
I guess it depends on what you use firearms for. 99.99% of my shooting is target, benchrest, or experimental; not hunting. That means I have more specific desired characteristics in powder I guess.