How many trail cameras do you typically have out per 40 acres to use for scouting purposes?

depends on variables....can you put out an attractant? if so then 2 should suffice. if not then i find the hottest sign(buck) and hang on it. might be 1 cam might be 5.
you will get different opinions but really it is goin to be based off of your property

Like snapper1982 said, depends on lots of variables. Use of attractants, funnels, how the property lays, how much you want to disturb the areas you are hunting, etc. I like lots of cameras. I split the farm up into 5 chunks. I have 10 cameras, so I will take one of those 5 chuncks and put about 5 cameras on it at one time. Like 150 acres or so at a time. Leave it for 2 weeks in the preseason then pull them all one day and move to the next chunck. Find my 2 biggest bucks on the farm, then I stay out!

Depends on how much land you own, control with a lease, and the time available to survey and scout the land. Depends on how much available funds you have. Depends on how many deer plots you plant.
If you were a hunter using only public land, at best, you might want only one, fairly inexpensive scouting trail cam. And take the risk of having it poached.

Heck, I know one professional involved in deer hunting, who files a federal corporate tax return, and deducts the cost of all his trail cams over their useful life. He has a different basis on how many trail cams he uses. Most of the ordinary hunters can't even think of competing with him. And never will.

1 maybe 2. Depends on what all you got going on the 40acres.
How many foot plots, or watering holes.
Just depends on the terrane.

I have 33 acres and 2 trail cameras. I do see different deer at each spot.