I am building a 3 walled shed under a roof off the side of a building. I want to use 3 courses of 4x8x16 hollow concrete block. There is already a concrete pad (8"thick"). The lumber/steel siding (barn siding) wall will be 10'6" on top of the concrete wall and will secure to an already existing roof that is overhanging off the main building.

I want to know how much vertical rebar needs to ran through the blocks. Like 1 in every block etc. or should I drill holes in the concrete floor and run 1/2 in All thread into there so I can bolt my 2x4 footer on top of the wall? I will epoxy the rebar/all thread into the concrete floor.

First off the 4" block is to small for this project. Second of all there isn't enough room/ core for bar and Crete inside a 4" block wall to amount to anything.

Use 6" block and keep the extra width inside the building, block wall flush to outside facade. To cap the block use a 4" solid. ( 4x8x16 ) They come with slots in the side for anchor bolts to pass through them. Normal bolt location is no less the 1' from any corner and every 6' or less from there.

The 6" block as enough room inside them to set an anchor bolt. You can run a stub pin in the slab to pin first course. Not necessary here for Code.

Re cap.
Lay 3 courses of 6" hollow block.
lay out pattern for anchor bolts. Stuff loose rock or empty mortar bags in block cores. ( Wedging in cores a couple of inches below the anchor bolts going in there.

Leave the last full 8" block hollow. To set anchor bolts flop some Mortar in that core. ( wedge spot) Plop a rock or a piece of broken block on top of that fresh mortar. Now set tape measure on top of the full block. Measure up from there 6 3/4 To 7". This allows for the 4" solid and a 1 1/2" wood plate and 1"or so for washer and nut.

Heres the important part. Always pre gage a bit and gently tap the anchor bolt down to get the right height. After its good, throw some more mortar on L part of bolt in block. take a small rock / broken block piece and lay it on the fresh Mortar. From there use Mortar block / stone pieces/ loose stone etc. to fill up hole core. Now lay the 4" block.

I lay My 4"solid cap block and set bolts as I go down the wall . Easier to to shift anchor bolt in web cores to adjust for 4" cap head joints/ vertical joints locations and set height on anchor bolts.

Now you have a solid foundation to build on and a good 4" whether proof cement cap to set plate and bolt down.

Much easier then what you were going to do. Just to put you at ease here, I am a Professional Mason.... Note: check codes for length of 1/2" anchor bolts. Some townships require bolts to be past/ below the last full 8" course you lay.