I reckon a 2.5 litre tin would be more than enough. For best finish, you should sand to at least 120g, no finer than 150g. Make sure any scratches against the grain are sanded out – they will show up even more once you apply the oil. Use synthetic bristle brushes or mohair (synthetic is fine) mini-roller. Buy one of those rolls of stockinette to rag off the excess, and help you get it into the corners. It’s only a door so you don’t need a thick coat. Don’t forget to oil the tops and bottoms of the doors too. 🙂 Personally, I don’t think it needs knocking back between coats, especially on doors, but you can if you want.
EDIT: make sure you give the can a good shake as solids can settle at the bottom. When it says “apply thinly” on the lid, it really means “apply thinly” – it’s a high solids oil so will take ages to dry if you slap it on thick. Satin is not really as satin as you think – it’s the same code as the old “satin-matt”. The new semi-matt is quite nice though.