There is no word like lader. If you meant ladder, the steps of a ladder are called rungs.
rung
A ladder has different amount of steps, depending on how big it is.
The rungs of a ladder are the steps. Unless it is a step ladder, then they are just steps.
18 steps
steps
rungs
11 rungs.
The steps of a ladder are called rungs.
Rungs
rungs
The steps on a ladder are called rungs (rung, singular)