Depending on scale and actual current draw: A lot or a couple of trains.
Size of layout is irrelevant. An 8 amp booster can't send the proper voltage any further than a 5 amp booster if wiring stays the same.
What does matter is train density per square foot. In addition to engines you would need to consider accessories: headlights, sound decoders, lighted passenger cars, DCC powered turnouts, etc.
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Each booster should have its own power district. If you have a physically large layout, then it's a good idea to locate the boosters near their power districts, so that the wiring runs from booster to track are kept short. This will minimize "resistive loss" in the wires, and allow you to get more power to the track. Also, using heavy-gauge bus wire (#12, for example) will reduce the resistive loss.
You can probably run a couple of dozen modern locomotives on an 8-amp booster. Note that I said "locomotives," not "trains." If you are running multi-engine consists, then you need to count each engine individually. Sound engines take more power than non-sound engines, and lighted passenger cars count, too. If you're concerned about the load, use LEDs for all your car lighting and headlights. They draw much less power than incandescent bulbs.
Use separate power supplies for your structure lights. They don't need DCC control. Unless you are running your turnouts from your DCC remote, you should put them on a separate line, too.
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When I got my 1.7 amp power cab three years ago I tested it with as many locos it would run. The total was 7 sound equipped and 6 silent locos running in consist. The real reason the breaker tripped was a derailment at a switch. Not too bad haveing 13 HO scale locos running on less than 2 amps. Plus the fact that 2 locos had the old Bowser open frame motor with soundtraxx decoders. Talk about power hungry.
Take your 8amp booster and break it down into as many different blocks as posible useing PSX breakers and run your layout.
Pete
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