How many trains can I (safely and reliably) drive at once? One.
You may think differently about your own abilities. Self-delusion is a great joy.
Until that train running around the loop unattended, whilst you run a second train, runs into the turntable well due to a mis-set turnout.
If it takes me five years to build the single loco to run that train, and effectively four re-builds during the process, why should I worry? I will be learning and improving all the while. I expect my layout to take between 10 and 20 years to build, after all. In the interim, there is the brass market and RTR diesels to keep me going, plus re-wheeled freight cars. All of these can be upgraded or replaced.
There is no rush to the finishing line, for there is no finishing line! The aim of a pastime is to pass time. If I can acquire new skills and a heightened sense of achievement along the way, well that’s as good as a hobby gets, isn’t it?