Yes, you would expect to see more monitoring probes in a controlled space as the size of the space increases.
You also expect to see more monitoring probes as…
- The temperature sensitivity of the product increases.
- The price of the product increases.
- The temperature differential between the inside/outside of the area increases.
- The reliability of the cooling system decreases.
In the end, it is a risk-based decision. There is no solid rule. However, more warning letters and observations are seen for not monitoring at all, than for not having a high enough density of monitoring probes.
Pick a density of probes that makes you feel safe and confident. I have seen people examine their mapping studies, decide a distance (generally arbitrarily) to say no product should be more than X meters from a probe, and then come up with a probe layout based on that logic. For instance, I know one distributor who wants all products (in ambient storage conditions) to be within 25 meters of a monitoring probe.
But for anything under 20 cubic meters, you are generally fine with one probe. It does not need to be at the warmest / coldest spots. But it should be placed at a spot that does take that data into account. Most hot spots/cold spots are inconvenient, expensive, or dangerous to monitor. Find a place nearby that is representative. In the end, almost every manufacturer puts the sensor near the door, out of the way of traffic, in good airflow, and out of the way of spaces that can be used for storage.