I've sprayed a car as an amateur three times now, and I bought five litres and used most of it each time, leaving around 1/2 litre for touch ups.
I've found Jawel paint to work very well and it's not expensive, especially if it's as part of one of their kits -
https://www.ebay.co.uk/itm/Cellulose...oAAOSwo4pYLsJH
https://www.ebay.co.uk/itm/Cellulose...EAAOSwal5YJG7D
Their advice was to use etch primer on a fibreglass shell then their high-build primer to prevent flaking at a later date.
They do pretty much the whole range of colours and have changed an ebay kit to an unlisted colour for me with no extra charge.
Good luck!