Probably 4 locos, but depending on how much current they draw, could be more, could be one less.
You yourself will not really to run 4 trains simultaneously; most folks might want to run two by themselves, but generally, the other locos will just sit off ot one side. YOu might get four trains running if you have 4 operators in the house, but you need more panels and throttles for that. Most locos will draw about a half amp per, more if stalled, less if just sitting, so that accounts for all the power coming out of the booster.
Kennedy