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That's not easy to answer. It depends on the frequency a user needs to work with QV.
A session CAL is quite expensive and only pays off, if the license is released by the users quite frequently (i.e. if the license it's locked by e.g. same 4 users all day, it's probably cheaper to buy named user CALs or even document CALs, if the user only requires to see 1 document).
I suggest that collect some statistics / assumptions on the document usage across your users, then meet with your QV sales rep or QV partner to discuss possible license scenarios.
Adding to Swuehi, If you have less number of report in your environment , then the Document CAl will be cheaper , you can buy a specific number (depend on your parllel user count) of Document CAL and assign the same to the respective document.