Typically, most invisible fence kits come with about 50 flags, but more can be purchased separately.
Here are a few factors that will affect how many flags you may need:
How close together you want them (typically we suggest doing them 6 to 10 feet apart) How large the boundary is (on average 100 flags can cover about 1,000 feet) Any extra areas that need to be blocked off such as pools, gardens, etc.
Putting as many flags down as your dog needs to start off is completely okay.
These flags will do their job of being a visual representation of their boundary and as time goes on, the flags can be removed.
Typically within about 2 weeks, a dog will understand their boundaries and the flags can begin to be removed.
You can remove them all at once, or you can do it gradually by removing them little by little and spacing them out more as the training evolves.
Regardless, starting with the ones available in your invisible fence kit can give you an idea of how many you may need for your space.