I believe they come with 3 and it is recommended you add 2-3 more for $150? each depending on size of planter. It has it's own "module" that takes signal out of your planters harness and does a much better job processing it than ANY current planter monitors do. I've heard of several guys saying they had a sticky chain link on rows at different times and it was going out of parameters and showing up on the 20/20 but was still within the type of error you'd ignore on the factory monitor.

I have no idea what the AirForce costs.