"launch the game simultaneously in several windows" several = more then two, with one premium you won't be able to open more then two clients. text is shown on the frontpage when you don't have premium, but it's false information so should be changed.
dear user, its clearly visible this advantage is available for premium account and the button is below. as for several dictionary specify also: "an indefinite but small number; some or a few: " regards
not according to oxford dictionaries: http://www.oxforddictionaries.com/definition/english/several it does show "several persons or things; a few; some." on other dictionairies but they also show it can be confused with "pair" or "couple". it would be much better if the description would say "launch an extra window of the game simultaneously", but i guess it would be too close to the truth