If Russia saves submarine second strike, it will be 800.
If Russia will chose to hold off land-based second strike, it goes to 550.
If Russia chose to reserve some of its most powerful ICBMs as a reserve for the "dire friend" China, I would give it to be 300 warheads.
If the nuclear command & control centre is destroyed by a pre-emptive strike, there will be no coordinated first strike, but uncoordinated launches by individual units, highly conditional on officers resolve to fight an now unwinnable war.