Tug, two keel blocks would be a minumum--I prefer 3. 4 jack stands are enough. 3 keel blocks and 4 jack stands allow you to move the stands/blocks to prep and paint the areas which are covered by the stands initially. I would generally do the entire bottom--then move the blocks/stands and do those areas not yet done. It is also possible to set the blocks so that you can reach the areas not done, once the boat is back on the trailer.
There is another possiblity--which I have done--and that is to lift one side of the boat on the trailer, block it--cover the trailer bunks, then roll on the epoxy and paint--not a lot of working room--and better for a "recoat" of bottom paint.
I have also painted the bottom of boats on my lift. Just set the boat on cross beams, instead of bunks--and them move those beams for the catch up epoxy and paint--if you know someone with a lift. For that you have to have some work platform or work boat to use under the lift and boat. Also I lock the lift with chains when working under it.
There is another possiblity--which I have done--and that is to lift one side of the boat on the trailer, block it--cover the trailer bunks, then roll on the epoxy and paint--not a lot of working room--and better for a "recoat" of bottom paint.
I have also painted the bottom of boats on my lift. Just set the boat on cross beams, instead of bunks--and them move those beams for the catch up epoxy and paint--if you know someone with a lift. For that you have to have some work platform or work boat to use under the lift and boat. Also I lock the lift with chains when working under it.