How much rebar should be left above concrete slab for the blocks to be laid

Kind of a different question the more the better in a perfect world. If you are just pinning the first course of block to the footer then 4" is fine then slush mortar in that core 4"

If you are going to continue that vertical run then 8 " and tie another bar onto that to the working height that's comfortable for you to lay block over.

What's this for?