How much gearbox oil should be in there?

Liz,
Ignore not a Psychic, that person seems to get more joy from telling people how daft they are than actually helping.
Firstly, as CDC says, the fact that your repair people don't know is a concern. If however, you had a friend do this and they genuinely don't know, then get them to take CDC's advice and Google it.
Help the contributors by giving as much detail as possible.

Oldgrubber
April 2017

We aren't sure how much oil to put in either, as you didn't bother to tell us the make, model and engine size of your car.

Not a psychic
March 2017

There are various sites available out there for access to all vehicle data and information,in my opinion if a workshop doesn't have this for repairing and diagnosis on modern vehicles then I would suggest you find another garage to repair your motor.

cdc
March 2017