As Texastrain suggested , the answer is relative to each person's reality. For me, the limit became not so much a number (which I will not disclose) but a feeling. That is, I started buying locomotives and rolling stock and building the various iterations of the layout in 1995. I rarely missed a trainshow in the NJ, NY, PA, Conn. area. Once I slowed down in my purchases, my wife started buying me locomotives, for each birthday, anniversary, and Christmas.
For example, during a trainshow in Edison, NJ, she spontaneously (no special occasion was imminent) bought me a scale Williams "Challenger" and a scale Weaver UP8444. (Yes, I was delirious !) After that, I tried to keep my enthusiasm for drooled-over treasures a bit less apparent.
I actually reached a point, when she bought me a scale Allegheny steam locomotive at our LHS (the proprietor had been observed, over the years, nearly "snapping to attention" whenever she came through the door), when I asked her to stop the very generous kindness . She reluctantly agreed.
I had simply - quite unexpectedly - arrived at a point where I had enough trains, with plenty to run, which are very dependable, plenty to admire on shelves, and plenty, altogether, to have pride-of-ownership galore.
Making a nice layout became my sustained passion, instead of acquiring trains.
FrankM