Can be achieved by placing different Rugs, it doesn't matter what type (can be kitchen, chairs, art) as long as it has the Rug indicator in the game.
More Rugs will unlock while researching different things at the decoration shrine and you will also be able to unlock some Rugs via friendship levels.
The game will state if you already have 1 of an item and that the stats won't go up, i.e.; have 2 sorts of Rugs, Rug: 2 stat, is not 4 stat, it's 2 stat.