i found a blog has 20+ pr 5 pages. If i post a link on all 20+ pages will that give me extra links juice or wont matter ?
Sometimes. usually they will show up for the first few backlinks updates, then they will drop off until only 1 or 2 show up.
I would say post on all of them - even if Google only ends up "loving" one or two - at least you know for sure you'll get them!
Tease... Share the link love... lol j/k. If onsite link strategies are important than any link is important. Even multiple coming from the same site really. Congrats on a good find!
I will help but it would be better if your links would be spread to different domains or different IP range.
Link is a link. You will still get the benefit of PR5 backlink but the value won't be as high as getting from unique domains. It is good if you can get all PR5 from one domain, don't let go this chance either.