To determine the amount of electrons in a given charge we need to remember that the charge of an electros is:

coulombs.
Now, if we have a given amount of charge an we want to know how many electrons are in the charge we divide it by the charge in the electron. In this case we have:

Therefore we have:

We know that for each step the worker collects -50nC, this is the same as collecting:

electrons. Now, that we know this we divide the maximum amount of charge the component can take by the electrons in each step:

Threfore the worker can take 3 complete steps before touching the component