score:8
Unless you book the whole room (ie pay for all 6 beds), then yes, you may well be sharing with others.
Of course, depending on the time of the train, the service, and the route, and the time of year, it may not be full, and you may luck out and get the whole room to yourselves.
No, there's no 'partial occupancy claims full room' rule - if there's a spare bed, someone can buy that ticket and use it. You could perhaps try asking new people to use a different room, warn them that you snore or something, but otherwise no.
Six is not so bad, try platzkart class in Russia where you have 54 beds in one big open carriage, or obshi class in Uzbekistan - no beds, and 100ish people just sitting for 24 hours(!).