The WRobot subscription for only private servers, lasts for one year and costs 19.99€.
It supports the following private server versions: Vanilla 1.12.1, TBC 2.4.3, Wotlk 3.3.5, Cata 4.3.4, MOP 5.4.7 / 5.4.8 and WOD 6.2.3 / 6.2.4
Once you got a subscription, you'll be able to see your license key here http://wrobot.eu/clients/purchases/ and can enter your license key in WRobot.
WRobot runs then fully in background and you can run as many instances of WRobot, as you want. ( only from the same IP address )
If you don't want to renew the subscription every 12 months, then you should buy the unlimited subscription for 10 years.
The unlimited subscription, also supports the current retail version of World of Warcraft.