I think the more personal the blacklister is higher the number. Keen is like 7 I think and he's like the husband. The more the person knows about Red the higher on the list he is.

 The bigger threat the person poses and the more conecterd he is to Red or anyone close to him the higher he his.

I have a theory that number #1 will be the last episode and the name will Be Reddington because only he knows all the secrest that he has and only he knows him better than anybody else. Only he knows everything about the past. You get the point, but at the moment we just don't know.