Ideally you'd want two.. I have one sensor at or around canopy level and one sensor lower in the tent. The Canopy level is the best place to put it as this would be your "median" or your middle ground.. Don't put it directly under the light but at canopy level should give you a indication of where your plants are at.
I use 2 in a room one right above the canopy away from direct light and one at substrate level to also measure how well air is circulating is another benefit.
Good luck happy growing 🌲🌲🌲👊