PSLV-C12 (C stands for commercial) rocket that flew in April 2009, ISRO jumped one number and called its next rocket PSLV-C14, which launched Ocean-sat-2 and six other foreign satellites in September 2009. Following that, several PSLV rockets flew and ISRO launched its first navigation satellite Indian Regional Navigation Satellite System-1A (IRNSS-1A), named its rocket as PSLV-C22.
The PSLV-C23 rocket is slated to carry a foreign satellite later this year, preceding the Mars mission, with a rocket named as PSLV-C25. Till date ISRO has flown 21 commercial and three developmental flights of PSLV rocket. If one takes into account also the developmental flights, then ISRO is right to term its Mars mission rocket as its silver jubilee mission. The rockets are numbered based on the missions, and work will progress based on that number. Once in two years, the missions decided and gave numbers.