A number is only unique within a station and a year
Every police station numbers its FIRs from the start of each calendar year. FIR 0250 exists at hundreds of stations, and again in every year the station has been open.
So an FIR is identified by three things together: the police station, the year, and the number. Any search that finds one record needs all three, which is why a date range is a required field rather than an optional filter.
Padding
Portals usually write the number padded to four digits, as 0250 rather than 250. The padding is presentation, not part of the number.
FIRSetu accepts either. Searching 250 finds 0250, so you do not have to guess how many zeros to type.
Finding a record without the number
The number is a convenience, not a requirement. Searching a station over a date range returns every published FIR in that window, which is often the faster route when you know roughly when something happened.