What are you aiming for, the Guinness world record ? or matching the prototype ?

For a single locomotive, you might get ~20 standard box cars (~240m real world train length).
Most freight trains in the real world are 1.5km long which is 6.8m in Z-scale, that's a very big ask.

If you want to pull more cars, add lead inside the shell to increase weight and add traction wheels to locomotive.
With this, you might be able to pull 30-40 cars.

If you want even more, a double header locomotive would be good.

As a general guideline, 1 locomotive with added weight and with traction tyres could pull ~1m of cars reliably.

Test train lengths on curves not straights, they add to rolling resistance.

I run a single locomotive without any extras, and pull 600-700mm of cars. That's pushing the stock model.
Most of my freight cars have 2 axles (less rolling resistance), passenger cars and some heavy freight are double bogie (4 axles) = more rolling resistance.
I hope you don't have any inclines.