Skip Eastwood and buy real paint from a real paint store. Eastwood doesn't make paint, last time I checked...
Don't buy the 2nd tear paint like Omni because it's [censored] thin and you will end up spraying more coats to get coverage and end up spending as much money as good paint, not to mention how much more time it takes. BASF, Sikkens, Dupont/Axalta and others have 2nd tier [censored] thin paint too, so don't be all offended I dissed PPG.
I was in the paint business for over 20 years and can assure you that there is not such thing as good, cheap paint.
To answer your question about how much to buy; it all depends on the quality of paint you buy. The real paint jobber store will be able to tell you how much you will spend to paint your truck and then hand over enough, after you pay them.
They will also give you over the counter solid advice if they are worth their salt.
Tip: talk "sprayable quarts" because that is what really matters... not what the mix ratio is. 4:1:1 and 5:1:10% is irrelevant when pricing it out.
Tip#2: buy more than you need because if you run out and have to buy more, it won't exactly match the first batch.
