Let me play the math game here, this is more backwards in setting some preferences for combat first, and then going from there.. So how many 1st level people can a 10th level person take in this idea of a system (4e similar)
Let's say that level and abilities are the main separators (both have the same equipment ie Enhancement bonuses to attack do not exist!)
Attacks
1 always misses, 20 always hits (ignore crit multipliers/bonuses)
Lets go with a system that averages a 60% hit rate on equal level.
I prefer bonus every other level, since it keeps things from escalating. So being 9 levels higher results in a 40% increased chance to hit, and a 40% decrease in being hit (+4 atk, +4 defenses over lower, 8 point difference)
So the higher level person has a 95% chance to hit (don't roll a 1), the lower level person has a 20% chance to hit. So for every hit I get on him, he'll have to swing ~5 times.
Hit Points/Damage
Hit points, I'm a fan of front loaded hit points to prevent easy one hit kills on low level players. So if our hit points per level are x.. then 5x at first level, and 14x at 10th. So 3 level 1's have an equivalent number of hit points approximately. Let's say three swings should be enough to kill an equal level person. So level 10 deals 4.7x damage, and level 1 deals 1.7x damage...
So a level 1 needs to hit the level 10 person 8.24 times, the level 10 needs to hit a level 1 once.
What do the numbers mean!
Unfortunately abilities are in the air, at this point I'm assuming the level 10 hasn't gone out of his way to get the above numbers...
42 levels 1's have a chance of bringing him down in the first round. So how many can he drop before they get their 42 swings... About 9 (they're dying off 1 per round, so their damage drops each round). This is playing that he takes a swing, and all his living opponents take a swing.
So if he gets multiple attacks, or has some cleave method, or even abilities to negate attacks, or heal/shrug off their damage he's further above that. So let's say ability wise, he's twice what they are..
My vote is 20 (ok I'll vote 30)!