Two...one for the living room and one for my room. Buy one based on what size room the purifier says it's for. A good one will run you at least 80$. I bought mine at lowes for about 70$ (on sale) last spring. The smaller ones work fine but generally the larger ones are made to catch the smallest particles. And I would get one that uses <i>HEPA</i> filters...<i>personally I don't trust the ionic stuff works that well</i>. I read some awful reviews on <u>consumer reports</u> about ionic purifiers. <img src="i/expressions/face-icon-small-happy.gif" border="0">