how to make decision based on users reports
You don't need a prediction model for this. Maybe if you have had users' data. But without anything else, then you just need labeled data. Historical data that you know if it was a real accident or not.

When you have your labeled data, then you can follow a process like this, which is still heavily dependent on the kind of your data.

Start iterating on your labeled dataset and calculate the accuracy of a real accident's report for different combinations (5, 10, 15, 20, 25, 30 ... mins) and (1, 2, 3, 4, 5, 6, 7, etc users).

You will have a 2D matrix. I guess, acting fast on an accident is important in your case. Set an acceptable accuracy and choose the combination with the smallest interval, above that accuracy.