The requirement and calculation of minimum is highly subjective. Only in peacetime, the requirement us minimal. In wartime, Indian ships may also have to carry enough missiles to strike land targets and also defend against land based AshM when it has to also give cover to land operations.
If we have to go near land targets like Karachi, the number can be pretty high
Enemies can build many boats, even small ones like speed boats with people carrying ATGMS. Even these boats at distance of 7-10km can be highly problematic and have to be neutralised using some other means like helicopter based ATGM or even AshM missile.
So, the number of missiles including SAM, surface missiles, torpedoes, helicopter reloads etc will be enormous in case of major conflict. The requirement will be in 250+ easily per reload. It may go up much higher to 500+.
Even with high confidence, the number must be high in highly dangerous scenarios. Only peacetime operations can be considered less intense and hence less dangerous