How Many Pages are Necessary...

All business websites must have these at the very least:

Home
Services/Products
About Us
Contact
Terms of Services
Privacy Policy

Beyond that depends on need:

FAQ
How it works
Support
etc.

well the smartest answer IS 'it really does depend' on what additional functionality you give your users.. Let me tear one of these ideas of your apart and show you how it really does depend..

Sign Up/Register Page

- do you need an information page to tell your customers to sign up (1 page)
- sign up form (depending on whether this is a simple one step process or more (1+ pages)
- sign up confirmation, are you going to drop a label saying hey you're signed up on the -- sign up form or direct them to a new page (another page?)
- what happens if there is an error in the registration back end, where will you show this (another page?)
- are they going to need to confirm their sign up via email (1 - 2 pages)
- which page will they land on? (another page?)
- what happens if they forget their login? (1 page)
- and if they want to cancel, will you give them a confirmation account cancel page? (another page?)
- where are you going to calculate what to do with this information (another page?)
- how will you access the database (another page?)

as you can see page counts will depend entirely on how you scale, what you offer and ultimately these days ready made plugins, how many pages you will use depends on how you build the app.

Find a privacy policy generator online and once you've completed filling out the details you should add a menu item (page) for that on your website. I also suggest a contact page with a user submit form, an "About" page, and a sitemap (HTML) - which you then submit to google webmaster tools in the form of an XLM sitemap as wellm97 stated.