In a pond that size (depending on fishing goals) you need about 5 non- shoreline brush piles/pvc groupings integrated with your bottom structure (cuts ,humps, ditches etc ). Each structure may and often is made up of several items to make the one structure grouping. See the archive for ideas.

http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92463#Post92463

If you are using the added structure other than for fishing purposes then it depends on the goal/use.