There are 10 engines available for the mk8 golf.
Should be able to look up your manual here:
https://www.volkswagen.co.uk/en/owners-and-services/my-car/manuals.html
Should be a sticker in the engine bay with the oil weight.
You'll have to refer to your owners manual for quantity or give us more information as this forum covers all possible Mk8 Golf versions.
Just measuring voltage with a generic multimeter doesn't really give a good indication of whether a cell is serviceable.
All batteries really need to be tested under load to determine state of charge/serviceability.