Really depends on what type of paint you are using bud. I go for humbrol enamels, as well as some floquil colours, and like the application and finish of the enamels. But others like Eric (Superjedi), has amazing results with acrylics. So that can mainly be a personal preference. In my experience, I find the solvent based enamels shoot very easily through the airbrush, and have a really great finish. Acrylics can still be applied with a nice finish, and have an easier clean up, so it really is up to you..
As for amounts, the Humbrol tins are only 14mL tins, so I generally find 2-3 tins for large coverage areas, and 1-2 tins for areas with less coverage. This allows me to have some leftover, or a little excess if I mix something not quite right, as well as allowing for extra paint for touch ups etc.. Do remember though, if you have never tried the paints before, you are better erring on the side of more paint, which will just give you the freedom to experiment with the best ratios of paint to thinner, which can vary a little depending on your climate etc..
The floquil bottles are much bigger though, and 1 bottle per needed colour will suffice easily. Overall, for a quality ROTJ paint job, you will have to spend a good amount on paint, and may often need to go back to the store at times. It will of course depend exactly on your level of detail added, but if you end up being like me, your paint kit will resemble the amount of tins you almost see in a hobby store, and your local hobby store worker will know you by your first name..
Good luck bud, don't hesitate if you have any questions