I've got a twin mainline, reasonably lengthy (round a two car garage), and I can really only cope with 4 trains at once. (two on each mainline) (continually adjusting each to ensure they don't catchup/slow down). I've tried more, but it's just not worth the constant vigilance. I've tried more, with no probs powerwise, and also double-head and have had 8 locos running using only 4 AMPS of my 8 AMP supply. How much I had left of that 4 AMPS, I do not know. Remember a loco just sittng on a DCC track doesn't use anything. (unless you have the light/sound on, of course). And 5 AMPS would probalby power at least 100 locos, if they are just sitting there with just the light on.
I do this all through JMRI software, so I have a throttle for each loco on the monitor, rather than "recall" on a single hand held controller. If you are using a single hand-held controller, then no more than two at a time can be coped with anyway.