On a traditional plywood climbing wall, t-nuts are used to attach bolt-on climbing holds to the wall. T-nuts are like a nut for a bolt, that is attached to the back of the wall permanently.
Before the climbing wall is built, 12mm holes are drilled into the plywood everywhere a hold is to be attached. A t-nut is then added to each hole from the back of the wall.
It is recommended to use 25 - 35 t-nuts per of climbing surface or 70-100 per full sheet of plywood. That way there will be lots of spots to move holds around or to add more holds later, especially if the back of your climbing wall is hard to get access to once constructed.