Well, sort of.

NK has more than one now, though you're right that it's not many.

googles

This is as of late 2022.

https://thebulletin.org/premium/2022-09/nuclear-notebook-how-many-nuclear-weapons-does-north-korea-have-in-2022/

This issue examines North Korea’s nuclear arsenal. The authors cautiously estimate that North Korea may have produced enough fissile material to build between 45 and 55 nuclear weapons; however, it may have only assembled 20 to 30.

However, they can definitely mess up South Korea if they don't mind losing a war. They have a shit-ton of artillery at the border within range of South Korean population centers, a lot of it in caves and bunkers. IIRC estimates are that it'd take over a week for us to destroy that, and in that time, they could cause a lot of damage in South Korea.

https://nationalinterest.org/blog/reboot/north-koreas-heavy-artillery-capabilities-matter-more-its-nuclear-199704

North Korea's Heavy Artillery Capabilities Matter More than its Nuclear

Russia can definitely hit the US first and and wreck the US. However, I'm not sold that Russia still retains second-strike capability against the US -- or at least that the US military believes that it necessarily does or will -- and that's a big change from the Cold War. The US has been putting a lot of resources into first strike enablers.

https://www.belfercenter.org/sites/default/files/files/publication/is3004_pp007-044_lieberpress.pdf

The End of MAD? The Nuclear Dimension of US Primacy

The major point here is that the US doesn't have missile defenses adequate to destroy launches from Russia's arsenal if Russia launches first...but may well have the ability to destroy all launches from what remains of Russia's arsenal following a US first strike. The reverse is probably not true of Russia -- the US probably does have a second-strike capability against Russia.

And it's a pretty good bet that the US isn't spending on that capability unless it believes it to have a role.

You have the changes to nuclear warheads to give them very precise detonation times that improves their effectiveness against hardened targets (like silos):

https://thebulletin.org/2017/03/how-us-nuclear-force-modernization-is-undermining-strategic-stability-the-burst-height-compensating-super-fuze/

That's irrelevant for a countervalue strike, but important for a counterforce strike, and in particular if one uses depressed trajectory ballistic missile launches from submarines, they can be coupled with short flight times.

Upgrading the hydrophone network, which is important for finding submarines and being able to kill them prior to them performing an SLBM launch.

https://thediplomat.com/2016/11/us-navy-upgrading-undersea-sub-detecting-sensor-network/

Work on conventional hypersonics. Unlike Russia and China, the US hasn't worked on nuclear hypersonics. Nuclear hypersonics are useful if you're worried about an adversary's ballistic missile defense capabilities being able to intercept your ballistic missiles. But the US has shown a lot of interest in putting conventional warheads on hypersonic vehicles. There are a limited number of reasons you'd want a very fast, hard-to-intercept, very-expensive conventional weapon. A first strike against nuclear weapons is one. Any nuclear weapon destroyed by a conventional one doesn't consume one of the attacker's nuclear warheads. They don't have a deterrence or second-strike role, because they aren't useful as a countervalue weapon. But they are helpful in a first strike.

https://en.wikipedia.org/wiki/Conventional_Prompt_Strike

Conventional Prompt Strike (CPS), formerly called Prompt Global Strike (PGS), is a United States military effort to develop a system that can deliver a precision-guided conventional weapon strike anywhere in the world within one hour, in a similar manner to a nuclear ICBM.[1][2] Such a weapon would allow the United States to respond far more swiftly to rapidly emerging threats than is possible with conventional forces. A CPS system could also be useful during a nuclear conflict, potentially replacing the use of nuclear weapons against up to 30% of targets.[3]

A shift to stealthy nuclear-capable aircraft and delivery platforms. These permit for strike without much by way of warning. Note that these have non-first-strike applications as well (though they can certainly enable such a strike).

As of this month, the F-35 is nuclear-certified:

https://breakingdefense.com/2024/03/exclusive-f-35a-officially-certified-to-carry-nuclear-bomb/

https://en.wikipedia.org/wiki/Northrop_Grumman_B-21_Raider

https://en.wikipedia.org/wiki/AGM-181_LRSO

Meanwhile, Russia has been lugging some oddball delivery systems out of the closet, like a nuclear strategic torpedo. That's useful if Russia is worried about the credibility of their existing second-strike capability in the presence of US anti-ballistic-missile systems.

https://en.wikipedia.org/wiki/Status-6_Oceanic_Multipurpose_System

Point is, if Russia doesn't have a credible second-strike capability against the US, then Russia can only lean on the threat of nuclear weapon use so far as leverage, because if the US really does think that Russia has a high likelihood of engaging in nuclear war, the US is a lot more likely than Russia to launch first, as it becomes possible to successfully perform a disarming strike. The "oh, look, I invaded Estonia, do you want to have a nuclear war over it" gambit, where one tries to convince the other guy that they're more-willing to have a nuclear war than you are, becomes a lot more dangerous for Russia, because the threshold for the US to say "yes" drops quite a bit relative to Russia's threshold.