Hi Marius,
You would need a minimum of two pair or four each. I would recommend looking at our new MK690TT speakers. These are dual application speakers that feature Dual Voice Coils and Dual Tweeters. They have a unique 4/8 ohm stereo/mono switch that allows you to connect the speakers as a stereo pair (connecting just one of the voice coils and setting the switch to 4 ohms). Assuming you have room (15 x 18 feet), I would mount each speaker 5-foot from the short wall and 4-foot from the long wall. The beauty of these speakers is the dual tweeters that dramatically increase the dispersion pattern of the high frequencies. I would start there then determine if you want to add a dedicated powered subwoofer like our PS88. Good luck!
TL/MA