Cato Boards can be used in a variety of ways, so the quantity you'll need depends on a few factors. While we obviously can't tell you how many Cato Boards to order, we can give you some things to consider.
How many dogs do you own? Most people with multiple dogs like to have at least one Cato Board per dog. This allows you to do group training with a spot for each student. When each dog has their own "spot", taking turns is MUCH easier!
How you plan on using your Cato Boards? Initial Cato Board training typically starts with one Cato Board. Once your pup is comfortable on their Cato Board and has learned to remain on it until given another command, you may find a second or even a third Cato Board will allow you to take your training to the next level. Multiple Cato Boards are a great way to teach directional work and handling. We find that once a puppy has moved beyond the "puppy" stage, we frequently end up using two or three Cato Boards in our sessions because of the added complexity the additional Cato Boards add to the session.
Where do you train? If you're doing a lot of training indoors but also training outdoors, consider having a Cato Board(s) in each location. The indoor one stays clean and easily accessible, while the outdoor Cato Boards make for an easy transition from the indoor training.
Consider how you'll be using Cato Boards in your training, and it should make deciding on a quantity easier. However many you purchase, remember, you can always come back for more later on!