Essentially, you have to meet a specific number of criteria for mild (5-6), moderate(7-8) or severe depression(9), but it can be fewer if you see a serious impairment in functioning or distress that requires a "heavier" diagnosis. That's true even for mild; if the person only has 4 symptoms, but the assessor can see significant distress or impairment, he/she can still diagnose it.
That's specifically for MDD. If it's a different depressive disorder, or your adding a qualifier, the qualifier may require something else (ie: for anxious distress, you only need two 'anxious' symptoms on top of MDD for 'MDD, X, with anxious distress, mild).