MichaelSomething
Legend
As well as any other 20th level barbarian with a 20 strength. Which is not at all. I think you want to play mutants and masterminds with Superman instead of dungeons and dragons.
Now that I'm looking at the encounter guidelines, a dozen T-Rexes at the same time would be too much for a 20th level barbarian. A 20th level character can handle 40,000 xp worth of monsters in day. T-Rexes are 3,900 xp each. According to the XP threshold table, fighting one T-Rex would be a bit below a normal encounter and two would be hard. So based on encounter guidelines, A 20th level character should be able to defeat 10 T-Rexes in a day if they split up the T-Rexes into pairs or singles. Now if there some favorable factors like good terrain, more short rests, magic weapons and the like, 12 T-Rexes in day is feasible for a single 20th level character.
So a dozen T-Rexes at the same time is beyond a 20th level character, but 6 T-Rexes in a singe day should be quite doable. Something like that could be in Conan's ballpark...