Am I using the correct concrete blocks for the base of my shed and how many do I need?
Personally, I would dig the topsoil out from under the blocks&mdash;preferably down to mineral soil so that almost all of the concrete is buried.  This makes less of a step up for wheeled equipment and less space for critters and weeds to grow underneath.  I put mine very close to the ground on concrete blocks with slots for 2 inch wide lumber and concrete screws through the joist into the block to keep the wind from lifting it up:



The kind of concrete block you are using is plenty strong, though I don't see anything to keep your shed from moving, for example from strong wind, heavy rain, or leaning livestock.

The strength of the joists is the next weakest link in this design.  Some more support in the middle could be overkill, but it is unlikely to be so.