Are you using DC or DCC? Small power packs, like those that come with most train sets, don't have much extra power, so when you add more locos, everything gets slower and slower. A better quality power pack will run multiple trains with ease: you'll need to determine the maximum number of locos that you'd be likely to run at one time, then get one with sufficient power. I use a MRC Controlmaster 20, and have run over a dozen locos at once, with no real difference in their speed as the number of units was increased. Normally, four locos on a train is the most that's required, although, of course, the weight of the train will increase the power required.
If you're running DCC, I believe that there are booster units available, but beyond that, I'll have to defer to Members more knowledgeable in that field.
Wayne
