The number of holes really depends on where you are located. I would say maybe start with about 5-6 evenly spaced holes on the front and back. Set the tub up with heat, water bowls, etc, and watch it for a day or so. If the humidity is too high, then you can add a few more holes, until the humidity comes down to where you need it. What works for me is making two rows of holes on the front and back. I space the holes about 2 inches apart. Most tubs are small enough that placing the holes lower on one end and higher on the other is not needed. That is something that usually works for larger custom melamine and PVC enclosures.
Since most tubs are opaque and not clear, it is usually not necessary to black out the sides, unless you want to.