It’s not the number of nukes that is important, it’s the number of weapons platforms to deliver them. Currently, that means Vanguard class submarines of which we have four. Why four? To guarantee at least one boat on station in the Atlantic at any one time, while one is in long-term maintenance, one in dock and one on at-sea training or on passage to and from station.
Clearly, if the defence strategy is to increase nuclear capability then a single boat is vulnerable to counter strike. So the best response is to have a second boat on station, which would mean a fifth sub. The next generation of subs succeeding the Vanguard class will be similar. However, long-term, and we are talking 50 years ahead, it may be that the sub platforms are much smaller, there are more of them and they are unmanned. Subs will be going the way of aircraft in being replaced by unmanned drones. It’s the human life support systems and the sailors themselves that are the main restriction on operational capability.
Unfortunately, the move towards unmanned warfare makes war more likely than not, imho. But these disputes will be carried out at arms length, in the oceans and atmosphere, but also perhaps in poorer client states as recently, where the Middle East and North African have been the “playground”. While there are nukes, major strikes on the homeland of nuclear powers are unlikely, and the aggression is at the sporadic terrorist level. But even terrorists are funded by someone and that ultimately finds it way back to some form of renegade state funding if you dig deeply enough.