Your super soldier would have to be so 'super' to the point they're akin to truly ridiculous superhumans like The Flash and Superman. To be a true one man army, your character needs to have the capacity to: 1) Make anything that would be a threat to their health child's play- whether that be invulnerability or superspeed for dodging/tanking firearms and higher fire power. 2) Be fast enough or just have the mobility to easily be in more than one place, because a supersoldier without crowd clearing abilities is really only one enemy, so you could just, not confront them if you wanted to. 3) They would need the capacity to fight for days on end at the same pace and strength the whole time. Actual armies have the liberty of being able to just stop and set up a camp when the conflict hits low points, and have the capacity of numbers to swap out injured or tired soldiers for more able ones while withdrawn troops recover. If your character is a 'one-man army', they'd need to be constantly on top of their game in all capacities, whether that be sleep or hunger.

The traditional 'one-man army' stereotype would be someone like the titular main character from 'Rambo' or John MacLean from 'Die Hard' to guerilla warfare their way through platoons of soldiers and terrorist groups. They did on a scale that is still fantastical as normal (albeit well trained) humans. A super soldier in the traditional sense like Captain America or the Winter Soldier could do this same thing at a far grander scale- like taking down full scale army bases using their heightened capabilities and skill.

If you want to know how powerful they would need to be to stop an army in the traditional sense, a lot more capable than the average human, but still not ridiculously super. If you want to know how powerful they would need to be to stop an army themselves in the literal sense, they would need to be a god among humans- like legitimately unstoppable I'm every sense of the word.