The real answer is... it varies. Early in Remix, one-shots were a major limiting factor for non-tanks, though that was largely solved by massive buffs to damage dealers and healers. Since then, surviving boss mechanics is viable with much lower amounts of versatility, though healing that damage back up and killing mobs before they wear you down starts becoming a more crucial factor.
How Much Versatility Do You Need to Solo Remix Keys?
Unfortunately, there's no specific number, since it varies considerably with class and spec, as well as the content. Some dungeons, like Maw of Souls and Vault of the Wardens are much more difficult than others, while affixes like Sanguine and Necrotic introduce their own challenges limiting long-term survivability. Certain specs are also just more capable than others in dealing and healing varying amounts of damage, with tanks now being particularly disadvantaged.
More power is always better though, and practically any activity will contribute to increasing your overall power level, leading to a greater likelihood of success. There are four breakpoints of rewards from Legion Remix keys, though, so we've come up with some very rough guidelines for each: some players will need more, others will accomplish it with less, though you don't need to solo the dungeons to earn their rewards either and there have been many groups offering free carries just to help players earn all of the limited-time rewards.
Disclaimer: These are very rough solo guidelines that will vary based on class, spec, the dungeon, affixes, and personal skill level. Tackling each challenge with a group can also lower the recommended amount of versatility by as much as half.
+20 Portals
Completing a +20 awards a personal teleport to that particular dungeon, just like a normal Mythic+ season. This is the only reward which necessitates completing multiple dungeons, though you may have already earned some of the teleports in previous seasons.
Beyond 700ilvl, this has a fairly low bar, and should be a fairly easy solo with ~50% Versatility by practically any class.
+30 Transmog
+30 awards the Timeworn Keystone Hero achievement and the Ensemble: Sargerei Commander's Hellforged Regalia. This is where you really want to start having closer to 740 item level gear, which can be farmed by using Mote of a Broken Time earned through a variety of activities, including rare mobs and daily lockout raids.
Above 100% Versatility is preferred for DPS, though tanks and healers may want to get closer to 150% to keep up with some dungeon damage requirements.
+40 Title
+40 awards the Timeworn Keystone Enthusiast achievement and title. This will be the big hump for most players and you should definitely have near-max item level gear at this point. Remember that excess Motes beyond ilevel 740 can be scrapped for more Infinite Power, so use that to keep pushing Versatility!
Aim for 350%+ Versatility to solo +40, though at this point how well your class handles various affixes can make a big difference in difficulty, so this could very well push towards 400% or more.
+49 Feat of Strength
Completing a +49 awards the Putting the Finite in Infinite Feat of Strength, representing the final reward of Legion Remix! Key scaling makes another big jump beyond +40, so finishing this last achievement will take everything you've got.
Soloing the final Remix challenge will likely require at least 600%+ Versatility, though depending on your class and spec this could just as easily be 700+. Doing it as a group is certainly preferred, not only to considerably lower the requirements, but also just speed up the process. Still, if you're looking for a challenge, this is it!
