I would be interesting in your criteria and in our decision-making process for this choice. I had one, several years back, and used it for all of one hour before I realized that it was a dead end for me. It would not allow me to use the decoder's full features because the EZ-Command is incapable of programming the various configuration variables (CV's) that the manufacturers of most decoders have designed to make our toys come alive and behave realistically. For example, when you dial in voltage on your DC system, your DC engines will speed up commensurately. With DCC, you should be able to change CV3 (Inertia), to make the engine appear to struggle to get its load underway, and to then speed up to your throttle setting. Conversely, when you close your engine's throttle fully, no train screeches to a halt in two seconds. It will take some time...and CV4 can be set to do that.
Having said all that, many folks have used the EZ-Command and been grateful for having a DCC system at all. I get that. But it would be a lot like getting a Ferrari with a governor that doesn't let you get above 25 mph, or one where you can only open one of the two gull-wing doors.
As far as I know, to answer you about the number of engines you can run, it is a 1 amp system...although Bachmann may have other boosters or power supplies for it...dunno. With simple, non-sound, decoders, you should be able to run between 3 and 5 engines without problems, providing you aren't making any one of them pull 20 cars or somewhat fewer up a grade over 1.5%. As you make any one engine do more work, you limit what is left over for any other engines...that should be clear. Now, if you are interested in a sound decoder, they typically will draw around 0.3 amps, motor and decoder, with the motor pulling a few cars. That means you may...may ...be able to run two three (3) decodered engines at a time pulling 5-10 cars each. Depends also on the condition of the drive mechanism...it is free and well adjusted, or is it older, and the lubricants somewhat hardened over time, or drive components misaligned....there are other conditions that can cause your engines to struggle. Tight curves are one. Grades we already mentioned. How free-rolling your cars are...and so on.
I hope I have not dashed your interest or your hopes, but I would like you to understand that it is a very limited system, both in terms of usable power and in terms of its overall utility in the grand scheme of all that DCC has to offer in a decoder.
-Crandell