So i live by a local park witch is 20 acres, obviosuly this question has no real anwser. But how many plants is a safe size in 20 acres with people who often walk the park....
None, if its walked often it will be difficult to hide and difficult to not get caught. Sounds like bad news bears right from the get go. Not worth the risk imo.
Not to mention, if it's a park, whether city/state owned or under environmental regulation, there's usually people who maintenance that park. Yeah, I can't see that ever ending in a good note.
A parklike setting is not a good place to grow. A forest of that size might have a hidden area where few venture.
Honestly you need to find a nice secluded spot deep in the woods where no one goes. If you must get on your hands and knees and crawl under thorns than do it. TRUST ME IT IS WORTH IT, it only takes one person to come strolling along to spot your plants. Also if planting a lot, dont do it in rows or any kind of pattern. DEA and police chopper pilots are trained to look for that.