When did you get this message? When you want to add users to the document, after you think you've set the number of allocated CALs to this document to 10?
If this is the case, then you have to click on Apply first after setting the value to 10. After that, you'll notice that the amount of CALs allocated to this document is set to 10 and you're able to select up to 10 users for this document.
Hopefully this will help.