Even if Ukraine only had twenty nuclear missiles and sixty warheads each consisting of around a single megaton, say bye to basically every significant Russian city.
Depends on the type of nukes.
Silos - not very useful to Ukraine, easily destroyed in a Russian first strike (that may not have to be nuclear itself due to hypersonic missiles, bunker busters, loitering munitions etc. being real now)
Ground launched cruise or ballistic missiles - they're already familiar with the Tochka platform, which is compatible with atomic warheads up to 100kt. Pretty limited range, may struggle to position these to threaten Moscow but can definitely threaten any future Russian troop/armor concentrations.
Air launched cruise missiles/dumb bombs - this is the old school SAC approach, went out of style in the 60's for good reasons. Could plausibly work against Russia, maybe, if they get the right planes (old European Tornado stock might be available and able to fly low under Russian radar before releasing the bombs/missiles)
Submarine launched nukes - the best all round option for nuclear deterrent. Ukraine has no submarines tho, their last one was stolen by Russia in 2014. They'd also need a good base for repairs and replenishment, and it's doubtful they'll have enough Black Sea coastline to make that work. They don't need expensive SSBNs, a small fleet of SSKs with launch tubes for atomic missiles would work.
Metal Gear - obviously this is the dream.
Trick will be obtaining the nuclear warheads, Ukie engineers are smart enough to cobble together a launch platform that would work but developing their own boom sticks is tricky. I don't think the US is gonna want to help them on that, and everybody else in the atomic weapons club has good reasons to refuse.
They might be better investing in drones. Lots of drones. Networked, autonomous drones that can make life a living hell for any invading force by constantly attacking troops, vehicles, and fixed locations even when Russians take down command centers and jam communications. The Turks are doing great work in drone tech, Ukraine has plenty of good engineers and coders who could take this to the next level.