I would conservatively rate each locomotive's current consumption at around 300mA (milliamps) max. That is the same as 0.3 A (amps) of electric current. All the modern models use less current but it is good to have a safety margin.
From that, it is a simple calculation. If your booster can continuously supply 3 A then 10 X 0.3 A = 3 A. So you can safely run 10 locos from a booster which can supply 3 A of current. Again, this includes some safety margin so in real life you could probably run more than 10 locos. That is because in normal operation at medium speed, a loco probably consumes between 100-200 mA (or 0.1-0.2 A).