You can fit more holes as long as you can drill comfortably straight down. The spacing between holes would be dependent on how you handle the drill. Too near each other, you will have overlapping holes at the other end of the wood, and you should avoid that. I can comfortably drill straight through if the spacing is 0.8" between holes. And I was able to fit 17" holes on the 3.5" x 3.5" wood block. I laid out the pattern on a computer so that it would be more precise and easier to manipulate. This way, I was able to add more holes according to the minimum hole spacing that I am comfortable with.
I cut the template and tape it to the smoother end of the block. Secure it unto a vise grip. Since I don't have a vise grip, I used our Christmas tree stand. It is heavy and holds the block very well.