He can probably handle two at once comfortably- they would only need to react between the three of them. As for the rest... I suppose because there is magic involved, and they are wizards, maybe it isn't so far fetched that they would stay back a bit in the fight. Remember that with all that magic flying around, it is dangerous to have too many allies in one area. Imagine Natsu trying to use his breath attack in a room crowded with both friends and enemies- it doesn't work.

I don't know what he is comfortable with, but I would draw the line with two people attacking him at a time.