We would always recommend having two thermometers in your aquarium.
This is to accurately display the ambient temperature in your aquarium.

Having a thermometer on either side of your aquarium allows you to more accurately measure the temperature gradient in your aquarium (how the temperature varies from one side to the other)
This increases the chances of you spotting any possible temperature issues in your aquarium before they become a severe issue.

For our range of thermometers please see here.
For our range of heaters please see here.