Could you elaborate on those basic rules [for distributed multisub placement]?
The basic idea is that you want each sub to generate a different room-interaction peak-and-dip pattern, and the sum of these multiple dissimilar peak-and-dip patterns will be much smoother than any one alone... and "smooth" bass is "fast" bass because it is the peaks (which decay into inaudibility slower than the rest of the spectrum) which make bass sound slow or boomy or unnatural.
So as a starting point, I'd suggest something like this: Place one sub in a corner (preferably a front corner), one sub somewhere along each of the two far walls (the walls "opposite" that corner), and any additional subs somewhere else but not too close to any of the others, and not in corners. Also not mirror-imaged or otherwise symmetrical with respect to any of the others. Bonus points if you can elevate one of the subs above the centerline of the room, to get significant distribution in the vertical plane as well.
If you can't follow some of those suggestions, then don't. You'll still be getting worthwhile improvement just by having multiple bass sources spread around the room.
And how many subs does it need as a minimum?
With three subs you can get pretty good distribution of your bass sources, so I'd say three is the minimum for a distributed multisub system.
And how many subs optimally?
As a general principle, the in-room smoothness theoretically increases as the number of distributed bass sources increases. (I use the word "theoretical" because in practice the improvement is never quite as much as the theory predicts.) So two subs are about twice as smooth as one; four subs are about twice as smooth as two; and eight subs are grounds for a divorce in most jurisdictions.
Seriously, I don't think there is a point where adding more bass sources is acoustically detrimental, but obviously there comes a point where more subs is impractical.
How many subs are the sweet spot regarding a good cost-performance-ratio?
That will depend on the specific situation, but I use four subs, which seems to be a "sweet spot" for me for three reasons:
First, my original target market was Maggie and Quad owners, and two monopole subs intelligently distributed have approximately the same in-room smoothness as one dipole source, thus my four small monopole subs are a good match for two dipole mains.
Second, my four 4-ohm passive subs can be wired in series-parallel to present a 4-ohm load which can be driven by a single amplifier (my Swarm system is aimed at a price point which is fairly modest by WBF standards).
And third, four small subs intelligently distributed seem to make enough of an improvement that my customers are happy. I'm sure my customers would be still happy if the Swarm had eight subs instead of four, but then I'd have a lot fewer customers in the first place!
Now IF you already have a good sub or two, then the sweet spot would probably be for you to add more. If you're "rolling your own" distributed multi-sub system, you don't need for all of the subs to go all the way down. So you can have one or two big subs, and the rest can be smaller subs which don't go as low.