You cannot add more than +1 right now. The default reputation of any regular user is 0,5% (1/200) of their own reputation, never lower than 1, though.
Moderators can actually bypass this limit and give reputation from 0 to 12 at their choice, and administrators can just give the amount they want, but I tend to follow the 0,5% rule.
For example, I've got 1100 reputation. 0,5% of that is 5,5, which approximates to 6, so I give +6 with each +rep.