I can't recall seeing a formula for traffic density vs. number of main tracks in any of my many railroad-related books. One that might have the answer is "The Railroad: What It Is, What It Does" by John Armstrong, which I don't own and which may be hard to find.
The easy way to sidestep any guessing is to either model a specific segment of a specific prototype--using the same number of main tracks as it did--or at least emulate the practices of one or more prototypes. If I was freelancing, for instance, a railroad that was set in the Northeast Corridor, I'd consider double, triple or quadruple-tracking. If I was building a model railroad set in the west, I'd probably stick with single-track.