One hole should reinforce the planing stop, as shown. The stop is clamped into the face vise. The hold down supports the end of the stop when working wider panels.
It’s easy to overdo it and drill holdfast holes that you’ll never use in your workbench. Or (even worse) you’ll drill holes in the wrong place, but only by a bit. My recommendation: Start with four holes in your benchtop and ponder any additional holes in your bench like you would a new body piercing.
Here’s the philosophy on the four holes: The first hole is on the end of the bench where my two metal planing stops are (which I don’t cotton to) and is 6″ in from the back edge of the workbench. This hole does a lot of things, but its most important job is reinforcing a shop-made planing stop that clamps into your leg vise. To drill the hole, position your stop in your vise so it is clamped where you want it. Then drill a hole that is tangent to the stop and 6″ from the back edge of the bench. My Veritas Hold Downs use a Ã?Â¾”-diameter hole. Check your holdfasts before boring.
The remainder of the holes are based on the reach of the holdfast. You want your holdfasts to reach all along the back edge of your bench with no dead spots. This allows you to clamp battens down anywhere along the back edge of the bench, which will support your work from the side.
The Veritas Hold Down has a reach of about 9″, so I subtracted a bit from that reach and drilled my holes on 16″ centers. All of the holes are 6″ in from the back edge of the benchtop.