Building my first first coal/coke break-drum forge. Seen a lot of blowers advertised as xxxCFM. How much is too much or too little for forging and or forge welding?

An old hair dryer is plenty of air. They don't need that  much. Also instead of  a brake drum, use a shallower brake rotor. I would also suggest a side draft instead of an updraft as they are simpler to make and use. Lots of info in the solid fuel forums.

If your not sure about the air hook a dimmer switch to your air source so it's adjustable.

You can always vent too much air. If you have way too much air, add a T to the line and blow the excess onto the blacksmith. (grin)