verb + かどうか is a grammar pattern that roughly means either of:
- (forming an embedded question) "whether or not"
- (forming an no-adjective-like phrase) "may or may not", "problem of whether or not", "whether-A-or-B situation"
In your example, かどうか is used in the latter sense. For example, 生きるか死ぬかです means "We may or may not survive" or "We are in a life-or death situation". Click the above link to see some more examples.
(literally) As for this settlement, even if we counted everyone, there may or may not be 15 people.
→ There are only 15 people or so at most in this settlement.
a settlement which has only 15 people or so at most
(literally) It's the deserted-ness to the point where there are only 15 people at most.