Yes three. I had two and bought a digital so I use them all. Also my exo terra is very deep (horizontally) so I test my temp in the front of the cage as well as the back. And there is a5 degree difference due to my lights being on the back of the cage.
And to the guy that asked what I meant by my heating pad being all the way up, yes I made a box with a dimmer switch with mar and female ends