Difficult to say, we don't really know how many they've mobilized. A report by the Russian Studies Programme by the Swedish Defense Research Agency in December last year estimated that the Russian southern military district could be expected to mobilize seven brigades with full air and naval support within any given week. A brigade is usually defined as between 3,000 to 5,000 fully equipped troops. Considering that Russia obviously had time on their side this time, that number's probably larger. Possibly closer to 10 brigades which is what the report expected to be mobilized after brigades were transferred from the other military districts.
The report estimated that Russia could expect to be able to mobilize a maximum of 19 brigades, 3 airborne divisions and 1 airborne brigade within a 6 month window.
The largest reason it takes that long is that logistics is Russia's achilles heel. They rely very heavily on being able to transport units by rail, both within Russia and within any eastern European country with compatible railway gauges. The report outlined that the ability of the Russian military to conduct sustained operations even in a limited conflict was questionable.
As for naval assets, they would probably play a minor role in the conflict. The Russian Black Sea Fleet only operates 5 larger combat vessels and 4 guided-missile corvettes apart from the usual jumble of anti-submarine units, coastal defence vessels and the odd sub or two. Enough to be a hassle but vulnerable, especially considering that Sevastopol is so close to Ukraine.
More or less the entire air force would be available, of course. But they wouldn't face an easy task. Ukraine has considerable anti-air capabilities, largely because they inherited the Soviet doctrine where USSR forces were always expected to be facing massive NATO air power. I don't have any exact numbers on them though but it includes everything in the old Soviet arsenal from mobile AA like Shilka to large, long range SAM units like TOR, BUK and S-300.
This is made worse for the Russians because of the fact that they lack experience operating in such an environment.
They managed to lose half a dozen aircraft or so in Georgia despite that conflict being tiny with only a hundred dead on each side. Suffice to say, I wouldn't want to be a Russian pilot going up against a fully prepared Ukrainian air defence.
That said, the Russian air force does have an intimidating stand-off capacity with a large number of advanced air-launched cruise missiles.
To put all of this in perspective: it's almost certainly beyond the capacity of the Russian military to pull off a Shock & Awe campaign against a prepared opponent on the scale of the one the US did in Iraq. They don't have that kind of experience. But they have more than enough raw firepower to make things very bloody very quickly.
