At low levels, your average magic-user is balanced against their pointy-stick-wielding brethren by being very easy to kill, so they need fighters to protect them. They sacrifice raw numbers – especially defence – for versatility.
The trouble with this is, of course, that if you’re versatile enough it becomes increasingly easy to compensate for any weaknesses, while those focusing on armour and damage dealt find they can hardly keep up with fireballs and force fields. Additionally, supernatural powers are often simply more useful – because the designers picture a mage of a certain power level doing things a similarly-skilled barbarian could hardly dream of.
This is a standard problem, and it ultimately stems from the fact that people have trouble imagining what a “mundane” fighter can do once they reach levels of ability that aren’t, well, mundane. Sure, they can hit harder, jump further, etc etc; but that simply doesn’t scale.
One way to make combat less boring and more creative – this applies generally, not just to this issue – is to have various maneuvers that can be used at a penalty. Thus, rather than doing more damage to some orc’s hit points, you might lop off his sword hand – or maybe, at an appropriately large penalty, his head.
But here’s something very, very common in pretty much every portrayal of hypercompetent fighters, that doesn’t really seem to show up to the same degree in games supposedly simulating them – holding off large numbers of opponents singlehandedly. Examples are probably already springing to your mind, here. But game designers have learned that extra actions are rare and precious things, so they tend to be doled out sparingly if at all. The solution, of course, is simple when you think about it: you want “fighting multiple opponents at once”, so just allow them to split their bonuses between multiple opponents. Depending on your core mechanic, this will almost certainly mean a penalty to each attack, since you’re starting with a certain basic level of competence for most tasks.
How much should this cost? Well, ideally, fighting two guys should be about twice as hard as fighting one of them. So if your system doesn’t feature exponential increase, then this isn’t going to get very epic, but then if you wanted epic then you might want to rethink that anyway. (Yes, D&D levels/CRs are supposed to be exponential.) Also, hitting one guy while blocking another should be possible, although the reduced complexity (and added simplicity! Reward your player for helping you do your job) means that cleaving through five men at once or whatever may be easier, if you want.
And, of course, one man against an army will need rules for “they can’t all reach me at once”, probably about equal to any cap you place on multiattacks. But really, that depends on your design goals, there’s no one perfect number here. Unless you’re using minis, in which case this is not even slightly an issue.