There is an odd-man-out problem. Nobody has ever tried to don a spacesuit without some help from people not in spacesuits. So even if they had enough, which they probably do not, it might not be possible. Getting into a modern suit also requires hours of decompression to adjust to their lower pressure. You can't just hop in and step outside.
(Mercury/Gemini and early soviet pilots were suited/dressed before takeoff. And Apollo suits were different than modern systems.)
This is very complicated. I was researching this as kind of a logic puzzle or scheduling logistics kind of game. Not just evac the ISS, but general ISS ops. This is actually pretty boring compared to the engineering challenge of working around ECLSS issues and the mysteriously complicated electrical system. Elements of lemmings and DF and simcity and scheduling game on the ISS. On a map of modules that is not a full mesh connected network, etc. How long can you improvise in face of disaster kind of game mechanic. This game idea went nowhere, mostly.
First of all the Orlan suits are just tuned up moon suits, continuous evolutionary change since the 60s no revolutionary change like the USA suits, and it appears two dudes have no problem dressing each other while in suits. I couldn't find anything one way or another WRT one dude jumping in a suit by himself or much about Russian suit training. I get the feeling it wouldn't be a problem based on past mission profiles for two astronauts to dress each other aka it's been done but isn't talked about much, just kinda assumed. That doesn't mean a 3rd unsuited helper would have nothing to do.
Sokol suits are available but they are not really built for outdoor use (thermal, wear and tear, sun visors, maybe harnesses?), you'd be basically immobile and probably stiff and uncomfortable after awhile. Doing an EVA in a sokol would likely suck pretty bad but would beat breathing space. If there's a hole in the capsule during re-entry (or takeoff) the sokol will keep you alive for a couple hours, although uncomfortably.
The USA suits don't fit thru russian airlock holes (holy cow! but true!) so you can only put the (two?) USA suits thru the Quest airlock. The Russian suits can go out Pirs airlock, although many believe it'll never happen, Pirs is supposed to be re-entered when the new russian multipurpose lab module is launched to replace it. We'll see if that ever happens. Russian suits historically went thru the Zvezda but its ridiculous, you seal the hatches and open a door to the outside, its not a "real" airlock. I was never able to get stats on which modules could "zvezda style" evac if necessary.
Where the suits are stored is a mystery vs where the disaster is (meteor hole or whatever). I guess if you need to get to Quest and there's a de-pressurized segment in the way, one of the Russians has to go out and fix it.
I never saw any pictures or written description claiming more than 2 USA suits in stock and 2 Russian Orlan suits in stock at any given time. I believe they have a sokol for every crewmember, three sitting in each re-entry capsule. The onboard suits are rotated and thrown out every couple years. The Russians got new Orlan suits a couple years ago with semi-advanced onboard system debugging computers, kinda interesting.
Decompression sickness is crazy analog and non-binary thinking goes over VERY poorly on this site, lets just say its a game of statistics, and if half of DCS symptoms don't even kick in for 6 hours (or whatever it was exactly) then given the 100% possibility of death vs maybe 50% some symptoms in six hours and 10% death, then obviously you risk it. If you have the most expensive people on the planet the furthest of our species from medical help and you're in no hurry, then you spend the full 3 hours doing the pre-breathe and the ISLE exercise protocol. In the olden days they slept overnight in the airlock but ISLE works and only takes 3 hours... Just spending a half hour in utter panic breathing pure O2 during a disaster in healthy young athletic bodies "probably" lowers the odds of DCS to about zero, or at least lower than the odds of whatever would make you panic EVA, so in practice its probably not an issue. You wouldn't screw around with DCS unless you were in severe danger, but you wouldn't have an emergency EVA just for the heck of it either. My point being if you die of DCS thats probably just a dice roll that something else almost got you anyway. Its not the main threat or even the most important threat. There would be no need to risk DCS unless you were about to die, in which case the odds of DCS not killing you are pretty good. Or rephrased DCS is not a realistic game criteria in my planned game that went nowhere.
"Ars consulted a number of sources to gauge the difficulty of donning spacesuits without any assistance from unsuited crew. Though none would speak on record, the consensus is that it would involve what was universally categorized as an extremely high degree of difficulty."
I don't know where to find a current inventory, but I think they now keep four US EMUs on the ISS. They've been dealing with various failures, and at one point were down to 2 working ones, but I believe there are 4 functional at this time: 3003, 3011 (the one that tried to drown Luca), 3010, and 3005. Dragon I think took up 3003 and returned 3015 during CRS-3.
(Mercury/Gemini and early soviet pilots were suited/dressed before takeoff. And Apollo suits were different than modern systems.)