If you have a planet that shows 49/1u in the detailed planet view, yes, that is what the planet currently has. A mini harvester should be able to take all of that in one trip. Just to be clear, when you click on the planet info icon (the circle) it shows Common Ore (let's say 45k/ 1k) the 45 is what is there NOW. The 1k is what the planet produces per hour. Same goes for Rare Ore.