sourav, look at this issue from China's point of view. I will leave Pakistan out for now, but will get to it at the bottom of my post.
If China destroys India's land based nuclear-tipped ICBMs and all her air bases, in a first decapitation style strike....will India still have the ability to respond with nukes? The answer is yes, because of India's SLBM-capable vessels.
Now how many Arihant Class vessels are there? Nobody knows, apart from the people who need to be aware. All OSINT folks fall back to satellite pictures to make inferences, but that is where it ends. Remember, the Arihant's first patrol was only announced after she completed her patrol. All things considered, it was a calculated announcement in which the PM himself attended. And it sent a message to all, that an Indian Navy SLBM submarine successfully completed her first silent patrol. The significance of that is huge.
Now if China sinks the Arihant, how many more vessels are out there? Which part of the Arabian Sea, Bay of Bengal or the larger Indian Ocean is China going to search to seek-out-and-destroy these additional vessels? Is it one more? or two more? or three more? Depending on who you ask, there is more than just one that has been commissioned. Some say, two vessels, yet others say three. But this very ambiguity provides that strategic deterrence that India needs.
By keeping the number of boats ambiguous and their subsequent commissioning, it complicates war planning for the Chinese. At the end of the day, the Arihant and her sister ships need to give the Chinese need a moment of pause. If I decapitate India, can India still strike back? And if the answer is yes, then is nuclear war worth the risk? The deterrence with Mutually Assured Destruction (MAD) only works, if there is a 100% guarantee that the other side will strike back. What is the point, if you are going to end up in the same fate as your enemy? The key word in MAD is Mutual. What you do to me, I can do the same to you. And from China's standpoint, that is a scenario they cannot avoid vis-à-vis India.
It is for this reason, that the exact number of vessels and their commissioning, are kept a closely guarded secret. If, when and where the Govt chooses to announce is the Govt's prerogative. Let the Arihant and her sister ships forever stay silent. What is the point in the aam admi - like you and me - knowing how many vessels are actually in service? And more importantly, why announce the exact number of vessels to the enemy? Since ambiguity serves the purpose, let it remain that way.
Pakistan is a different ballgame and consists of one main player - the army. Their parliament and their government is only for show. The real power broker in Pakistan is the Army. And their army generals are rational and sane human beings because of the lifestyle they enjoy. Corner plot bungalow in Islamabad, many serf to do their bidding, have a number of illegal hustles (business) on the side, real & living 72 on call 24-7, etc. Life is really good for the Pak Jernail. And he is not about to jeopardize all that over Kashmir or some other foolishness. He will send Jihadis to do the grunt work i.e. die, but he will not risk himself ending up as radioactive waste.
Remember Musharraf's famous announcement during Operation Parakram? I paraphrase, but it goes something like this...
"Even if one Indian soldier trespasses on Pakistani soil, they will witness a fire & fury like never before."
This was said to assuage the Pak public of the Indian Army's Strike Corps likely decimating the Pakistan Army in war. Pakistan soon responded with Nasr tactical nukes to counter the Indian Army's IBGs. But India called out Pakistan's nuclear bluff, not once...but twice.
1) Surgical Strike in 2016 by the Indian Army
2) Balakot Strike in 2019 by the Indian Air Force
In both, Pakistan did what it does best. Subterfuge, deception and lies to convince their people...that they won. To the Pak Jernail, demonizing India is a better long term strategy, than actually going to war with India. They are not about to start a nuclear conflict with India. There is no scenario for them, in which they will end up on top.