Do I need another booster?
"What size layout do I need before I need multiple boosters?"
"I'm a single operator, but I've got a 10x25 layout. How many boosters do I need?"
If you're asking those kinds of questions, you probably don't need another booster. Let's look at things a different way: Your booster has a certain amount of capacity, usually expressed by its current rating. For example, the Digitrax DB150 has a 5A current rating. It will power any combination of track until its current capacity has been used up.
Each locomotive, lighted passenger car, detected wheelset, etc... draws a certain amount of current from the track. Locomotives usually draw the most, with 1/4-1/2A being normal for HO Scale. That allows you to run (and have moving) 10-20 locomotives on a 5A booster. A single operator, or even two or three, is going to find it difficult to move that many locomotives at one time.
When you added lighted passenger cars and sound locomotives, you're likely to see more current draw. I don't have any rules of thumb to offer, but chances are good a single operator won't ever use all 5A of capacity at one time.
"How do I know?"
Measuring would be a good way. You can find designs for DCC ammeters that are inexpensive to build online, or you can purchase a meter like the RR-Ampmeter.
You can measure from the 120V AC side of the power supply, but that gets tricky. It will have to wait for another post.
Another way to know is just to watch your booster. It'll start shutting down or funny things will happen when you've got a lot going on. If your booster is getting hot, it might be time to give it some help.
"If I don't need another booster, what about power districts?"
The smaller they are, the better. If only one operator can't run his train, there's a good chance he caused the short and knowing this can check his train for incorrectly set switches or derailed cars.
It is much easier to wire around insulated rail joiners than it is to install them later. So, if you go overboard on the track divisions but still run everything off one or two circuit breakers you still have the benefit of being able to split the bus if you have to track down a track short.