Which licensing opportunities do I have for MSSQL 2012? 

First option: Purchase a "Microsoft SQL-Server Standard Edition Runtime Client License, incl. Server-Licence". One license per registered DocuWare user is required (no concurrent-use is possible).
Second option: Purchase a "Microsoft SQL-Server Standard Edition Win32 Runtime Processor License", you need only one server processor license. This allows unlimited DocuWare users and no additional SQL-Server Client licenses are required.