I want to make a round wishing well. I have 12 2x4's @1'h. What degree of bevel do I need so they fit into a tight circle?
As indicated in the accepted answer the bevel on each side of the 2x4 would need to be 15˚.
Assuming the 2x4s are standard dimensions of 1.5" x 3.5" your twelve 2x4s will make a construction that is just over 13" across. You may want to evaluate if that is the size you want.
If you desired a construction that was a bit bigger you could use 24 2x4s with a 7.5˚ bevel. That would yield a unit that was about 26.5" across.