If your building a traverse wall, see this page for more details.

That is a good question.  A little hard to answer though.  How many holds to have on your wall can depend on:

• your budget
• the total area of the wall
• the type of holds (ie sloper, edges, pinches, jugs)
• the angle of your walls
• your climbing ability
• how much or little variety you want
• number of t-nuts you have per sheet of plywood
• you get the idea, there are lots of variables

A very rough method to determine the minimum number of holds needed for a bouldering wall is to calculate the total area of your wall in metres squared, then multiply the total

• x 5 to 8 if you are just wanting to get started on a wall and have enough holds to climb on
• x 9 to 20 if you want lots of holds to train on no matter the size or style of hold

For climbing walls with ropes it varies depending on the difficulty of the route and the number of different routes you want per rope. 

The number of holds you need for a climbing wall depends on the people using it and what you want to accomplish in your training.  

The above formula may not apply to you or you might think it is out in left field, but you gotta start somewhere!!