Depends on a number of factors really. If I have a goal of 500 posts a day, then I would probably want 10 users who are consistently active on a daily basis, with an additional 15 or more that log in often enough to be still considered active.
There really isn't a specific number in my opinion. It's a subjective thing. Certain forums could easily be active with 5-10 members active each day, especially if it isn't the same members everyday. At the same time, having the exact same 5 members active everyday would generally appear less active.