Is there a limit to the number of holes bored in a joist?
If there is nothing in the code of practice which specifies a limit to the number of holes, then there is no limit to the number of holes.

From an engineering standpoint, this is because a beam with holes in it can act like a truss (see e.g. the bailey truss). The minimum hole sizes and minimum spacing is to allow enough timber to act as the vertical/inclined "truss elements". Therefore if it is ok to have two large, closely spaced holes at any arbitrary position along the beam, then it is ok to have holes of that diameter and spacing the whole way along the beam.