Modern N scale locos
Figure your modern N scale loco draws about 0.2 amps per loco. Figure a loco with sound draws about 0.6 amps per loco (or about the same as 3 non-sound locos).
You also want to leave some headroom just in case you end up with extra locos in the power block on occassion.
On my Siskiyou Line, a typical train has 3 diesels on the head-end and a two-diesel mid-train helper. I typically run one sound loco per consist. This means per train I have:
0.2 + 0.6 + 0.2 = 1.0 amps head-end
0.2 + 0.6 = 0.8 amps mid-train helpers
Total: 1.8 amps
Two trains per power district gives me 3.6 amps, three trains gives me 5.4 amps - or too much for a 5 amp power supply.
So I need to make my power districts small enough that two trains will be the max almost all the time.
The other reason to have more power districts is short management - here the question is where do shorts occur most often? You want to isolate shorts into their own power district.
Derailments cause most shorts and most derailments will occur at turnouts. You want to isolate areas with lots of turnouts into their own power district. In other words, a major yard or industrial switching district should be its own power district.
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