French and Brits can't really afford more nor would they being secondary figure in their relationship with US.The French, Brits and I believe Russian think it's enough.
As far as I know, the first two definitely have no more than one boat out at any time. Not sure about Russia, but no more than three I believe.
Having your 12 largest cities annihilated is plenty credible to me.
In general, I think nuclear arsenals should be about minimally necessary deterrence.
Look at Russia: the nuclear arsenal was the only part of their forces they maintained, soft of, in the 90s. And they continued investing after 2010 when the economy got better. Sure did them a lot of good in the current war.
Conventional forces should be the priority.
Also if US can get out of their current debacle and secure their hegemony for another century for the cost of "only" 12 cities (which would not be entirely destroyed by one missile mind you), who are we to say there won't be a government in near future that would gladly take it?
In my opinion minimal deterrence would only work if you're only secondary enemy to a more powerful nuclear nation a.k.a it only works when Russia takes most of heat. If Russia also goes minimal deterrence that would leave US as the ONLY nuclear superpower who can win a nuclear exchange overwhelmingly against the rest of the world. They might as well extort everyone else into nuclear disarmament.