There are two main options for how you can gain the required data:
1) Use Official Statistics: If the relevant authorities in your target area have a reliable system of collecting data on the number of affected people, use them.
2) Conduct a Survey: If there is no data available or if you do not trust the quality, conduct your own survey. Use key informant interviews (with authorities, local inhabitants, health staff, etc.) to identify the main hazardous events that occurred in the assessed area in the given time period. Subsequently, use the same method to get a reliable estimate of the number of people who were affected by these events (always cross-check the numbers from more sources).
Alternatively, you can also use both methods to maximize your data validity.