I noticed two mentionings of the number 8 when looking at the film.
1) Ennis speaking to Jack at the campsite about him not "seeing the purpose in riding a piece of stock for 8 seconds".
2) Jack mentioning to Ennis in 1967 that his son is 8 months old
I also noticed that if you add up Generation # 2 ( Ennis and Jacks contemporaries), there are 8 of them
Ennis, Jack, Alma, Lureen, LaShawn, Randall, Monroe, and Cassey.
Interestingly, adding up Generation #1 ( the parents of Generation #2 who have speaking roles in the film), there are 4 of them
OMT, Mrs Twist, LD, and Mrs Newsome
for what its worth,