Each individual airline has its own numbering scheme. However, there are some general guiding principles that tend to be followed.

Westbound/Northbound/Clockwise are odd-numbered while Eastbound/Southbound/Counterclockwise are even-numbered

Many airlines use certain ranges to denote certain regions of operations. For example Air India uses 1xx for NA/Europe flights, 2xx for Africa, 3xx for East Asia, 4xx for SE Asia, 5xx for Russia, 6xx for Domestic, 7xx-9xx for Middle East.

Yet other airlines incorporate the day of operation into the flight number on a certain route. Eg. flight 1011 on Monday, 1012 on Tuesday, 1013 on Wednesday, etc...