The number of pages in a treatment depends on two variables:
Since the goal of a treatment is to sell a script idea to someone willing to produce it, it's a good idea to understand the minimum number of pages it takes to tell your story in enough depth so that it stands out as unique and profitable.
Here's how you can start, and then make progress:
In the end, you may have about four pages in the bare-bones version, and less than 20 pages in the first re-write.
For both versions, use Courier 10pt font with generous margins.
When you pitch your idea, you'll have the short version to use as a 'sales aid', and the longer version to pass along when a buyer wants more detail.
Trick or Treatment has 352 pages.
The treatment of Armenians in the Ottoman Empire has 684 pages.
The Official Razzie Movie Guide has 380 pages.
The movie tie-in edition, released in 2011, has 288 pages (not including the pages with photos)
Things the Grandchildren Should Know has 256 pages.
235... but who's counting.
6 pages? bahahaha.
A CV should have approximately two pages. There are some people who just have one and it is fine.
Should Have Got Off at Sydney Parade has 304 pages.
The movie wasn't based on a book. It was an original screenplay.
You don't do homework like that. You do the project first, and however many pages you get is what you should do. Trying to aim for a certain number of pages is going to get you a sloppier project than if you just do your best on it.
How many pages should a Business Plan have?