I suspect the true answer to this question will be so heavily dependent
on variables that are specific to your environment that no one outside
your environment will be able to provide a meaningful response.

That being said, it's safe to offer a broad, general answer such as:
the number of employees you'll need is approximately:
(the number of items your dept receives) divided by (the number of
items a competent employee can handle in one hour) divided by (the
number of hours in a shift)

This is pretty straightforward, but gets complicated when you consider
that...
~ (# items the dept receives) may have seasonal fluctations or even
daily fluctuations within a week ie., December may be busier then
August, Monday may be busier than Friday, the day after a bank holiday
may be busier
~ (# items handled by a competent employee) varies by employee
~ (# hours in a shift) depends on potty / smoking / coffee breaks,
lateness, absenteeism, people going on vacation, having to bring their
kids home from daycare, getting sick, etc.

Other things to consider:
~ re-processing due to errors made by a previous shift
~ availability of part time workers to cover the partial person
generated by your math (for example, 3.6 FTEs (full time equivalents)
are required by the dept.)

You might work up some metrics like this and compare it to your actual
operation, then adjust the metrics as appropriate.

Good luck with it!