Short answer: four.
Long answer: four, and here's how...
When Creating a Classroom
Head on over to our article on how to create a classroom for instructions on adding products to a classroom as you're creating it.
Once a Classroom Has Been Created
If you want to add or remove products to/from an existing classroom, go to the Manage tab of that classroom and scroll down to "Class Settings."
Under "Content," you can see which products are already part of your classroom
To remove a product, simply click on the orange "Remove Product" hyperlink and click "Confirm" when the text box pops up.
In order to add a product:
Click on the text box that says "Additional Products."
Select the correct license.
From the menu, check which products you want to add.
Click "Add Additional Products."
Remember that you can only add four (4) products per classroom. If you would like to make more than four products available to your students, you’ll need to create another classroom, invite the same students, and add the extra products there.