Question 252559
First, we list all spacing options:
(i) 1 ft apart gives 91 posts (90/1 + 1)
(ii) 2 ft apart gives 46 posts (90/2 + 1)
(iii) 3 ft apart gives 31 posts (90/3 + 1)
(iv) 5 ft apart gives 19 posts (90/5 + 1)
(v) 6 ft apart gives 16 posts (90/6 + 1)
(vi) 9 ft apart gives 11 posts (90/9 + 1)
(vii) 10 ft apart gives 10 posts (90/10 + 1)
(viii) 15 ft apart gives 7 posts (90/15 + 1)
(ix) 18 ft apart gives 6 posts (90/18 + 1)
(x) 30 ft apart gives 4 posts (90/30 + 1)
(xi) 45 ft apart gives 3 posts (90/45 + 1)
(xii) 90 ft apart gives 2 posts (90/90 + 1)
Second, if the posts are between 8 and 14 feet, how many should there be?
If 8 ft apart, then (90/8 + 1) = 11.25 posts. Of course we would use 11.
If 14 ft apart, then  (90/14 + 1) = 7.43 posts.
So, if your posts are spaced between 8 - 14 ft, then your number of posts should be between 11 - 7.