If I'm correct in assuming that every time the harvester goes from the mining planet to drop off the cargo and it's at full capacity (250u for a small harvester), then one should definitely be enough for a planet with an output of 1000u/hour because the harvester does a mining cycle every few minutes (when it's in the same solar system). I guess this gets more complicated once you are mining outposts that are several sectors away and you have to factor in the time it takes to get from your colony to the outpost. The time it takes alone is worth it to use a harvester with a higher capacity so you can fully utilize the output of the outpost.