bolou,
Welcome to the Forum!

You don't specify how many cars or how much track you'd like to add to your set.

If you're adding track, you might want to consider adding feeder lines from your  transformer to various spots around the layout.  This will help eliminate dead spots or low voltage spots.

If you're adding cars, add a car & try it, add another car - try it again, keep adding cars until your locomotive begins to slip then remove the car that caused it to slip.
Or
You can use a spring scale and measure the tractive effort of your locomotive and an arcane formula.

It's a whole lot easier just to add cars until the loco slips....

Dave