2X+1 , where X is what you think you need, would be nice, but most of us have to settle for something less. I do agree, however, that more is better.
The standard gauge part of my layout is a folded dogbone, which is essentially a roundy-round. The back part of it is concealed under a long narrow gauge branch that runs atop it, partially as a double deck. Yeah, I know, probably something I won’t do again, since it’s against the wall and all access is from the front or underneath, but it actually works and has been remarkably trouble-free, thanks to close attention to design and construction.
The part of the standard gauge main that is hidden has three sidings. Thus, I can hold up to three trains, for two in one direction and one in the other. The plan is basically to run a freight in both directions, plus a passenger train. There is a reverse loop available to turn the passenger train, allowing it to work both directions.
I also have an empties-in/loads-out arrangement that is double-tracked. I can leave a loaded and an unloaded coal train there.
So I have a total of five staging tracks. I can still run anything around the mainline. I can make-up and run a local, independent of the staging. And I can bring the other trains “on-stage” as desired.
Only half the roundy-round dogbone is visible, which makes it look like the single track main it is supposed to be. I’d like to have a separate staging yard, even beyond the staging I’ve built-in already, but it would take some major remodeling and giving up living space (you mean some people actually do NOT devote every bit of square footage to trains? ;>) I want for other uses. Even then, I would only be able to run trains in and out.
This leads to how I considered my own needs in measuring capacity during the design phase, which you may want to consider when estimating your needs. If you have hidden staging, or even staging that is moderately inconvenient to access, then you should thin