Let's assume we have system of detector which is composed of 7 sensors, which can identify particles. Let's assume we have 7000 selected events, where total number of hits collected from the sensors are 15000.
I want to find how the each sensor are detecting the particles. To do that I think I need to divide the total number of hits by all of the sensors by 7000 events.
The reason of doing this is to understand how individual sensor is responding to the selected events. Therefore individual rates are very useful here too. To do that I think I want to scale the histogram with 7000 selected events. How I can scale that?
Once I'm done with the scaling, I would like to find the rate of the individual sensors; which means
the rate = The number hits from sensors/ total number of selected events
You can calculate the rate directly from your histogram. Assuming the pattern holds (which is likely since you have a large sample size of $7000$), then the rate will be the same for each category of the number of hits. In other words, the number of hits and the sample size are proportional to each other.
If you scale the histogram down by $7000$ (divide each category by $7000$), you will get the probability that a certain number of hits is attained, since the sum of the probabilities will now equal to $1$. Don't forget to draw the bar for $0$ hits as well, since it is also a possible outcome.