Yes, You need to buy CALs
I'd say 50 CALs based on the below information from the above URL.
Windows Device CALs might make most economic and administrative sense for an organization with multiple users for one device, such as shift workers.
Somebody else will have answer about Office.
Like Orange hat said, the CALs for the server are for the server, so, No, the Win7 pro installs do not affect the CAL limits on the server...
And, like all Microsoft Software, you'll need a license for each device you install it on. A good solution is Volume Licensing - they cost a bit more, but allow you to use one medium to install Office, for example, on multiple machines...