oh gosh...I just did a full head of single braids on my natural hair. LOL . I was wondering the same thing because I did them quite small with hopes to keep them in for about 6 weeks. I wear wigs as a protective style though.
**waits**
It really depends on the condition of your ends. If you have thick, healthy ends, then your ends should be fine. If you thin ends, then I would add extensions. Extensions can be added at the root or towards the end of the hair.
When I use to wear cornrows, I had my braider add hair at the ends to give me extra length and to take tension off my edges.
The one issue you may find is in the takedown. I've found that I have to be very gentle and to be wary of letting the shed hair tangle up with the other hair. And redoing the nape about every two weeks is a necessity for me, or I get lint locks at the base, and they are very difficult to remove. HTH.