India is situated in a difficult weapons of mass destruction (WMD) region. The fact that both of its nuclear-capable neighbours—China and Pakistan—are engaged in a secret and extensive nuclear cooperation framework adds to New Delhi’s difficulties.
Therefore, to create a robust nuclear deterrent, India has also embarked on developing the sea-leg of its nuclear triad, which is believed to be the least vulnerable and most survivable deterrence platform.
With the speculated launch of the fourth ballistic missile submarine of the Arihant-class which is capable of carrying 8 Submarine Launch Ballistic Missiles (SLBM), India’s SSBN programme is maturing into having a credible second-strike capability.
However It is not enough, We need more SSBNs for a powerful and effective deterrence.
With only 24 VLS tubes, which can only carry K15 or K4 non MIRVed SLBMs and can't target China or Pakistan from Safer Waters of Bay of Bengal or the wider Indian Ocean, it becomes all the more important that S5 class of submariners are prioritised and inducted in big enough numbers.
Currently It is speculated India is planning to construct 3 S5 class of SSBNs which will have 16 VLS cells each however it is still not enough and we need 2 more S5 class of SSBNs because of the following reasons
1. First two subs of the Arihant Class are pocket boomers and can't provide even good enough deterrence and will most likely be only used for crew training and relatively short deployments
2. First two subs of the Arihant Class are technologically old and relatively noisy and thus may be retired earlier
3. The VLS cells on the Arihant Class are relatively smaller limiting them to only carry non MIRVed Medium Range SLBMs, which basically means they can only be employed against Pakistan.
4. It is widely believed that for a permanent deterrence patrol one needs 4 SSBNs , with at least one sub on patrol, one preparing for patrol, one returning to port, and one in maintenance.
For Reference During the cold war, France had a fleet of six Redoutables class Submarine to maintain a CASD of two/three boats
If India Wishes to have a credible Sea based Deterrence against China 4 S5 class are a given with the extra 5th sub providing deterrence against Pakistan alongside the smaller and less capable S4 and S4* subs of the Arihant Class.