I’ve put 4 coats on the exterior so far and have probably used a quart in the process. I bought 4 quarts and had about 1/2 from a previous project total to use up for the interior and exterior. I just opened up my 3rd quart and have the ICC and tongue-box to complete.
My understanding is the final coats are what provide the visual finish. I.e. you could use flat polyurethane for 3 coats but if your final coat is high gloss, you will have a high gloss finish. Similarly, your final coat provides your UV protection. Traditionally Spar Varnish (polyurethane) has a high quantity of UV blockers.