I should add the more amperage the more stuff you can run. It's hard to say how much you need. Some locos draw a lot more amps than others--USAtrains locos are notorious for using a lot of amps. The more amperage the better. I have a five volt meanwell power supply and it sometimes tended to trip its breakers when there were three locos running
Also if you are going to run track power, as I do, you need to take some steps to make sure you get good connectivity. If you just use the rail joiners the track cems wth, eventually you'll start to have trouble. You can:
1. At the very least put some anti-corrosion, conductive paste in the track joints. I used LGB conductive paste, but also used anti-sieze compound I got at the auto parts store. Squirt some in the track joiners before you join them. It makes a BIG difference.
2. Buy rail clamps and add them at each joint. Easy, fast, very effective, expensive
3. Solder the track joints together with a jumper wire, Very effective, cheap, time consuming and hard to do (for me anyway)
I've got I guess a couple hundred feet of track with rail clamps on most, but not all, of the joints and conductive paste on all of them. I rarely have any problems and when I do, it's the top of the rail, not the joint
another thing you can do is use all stainless steel track. It does not oxidize and needs less cleaning. But I don't find track cleangn in general a very big deal. I just run an aristo track cleanign car around