If you're referring to the first Saints Row then you unlock them as you progress in the story. Saints Row 2: You can purchase them and unlock them in story missions. Saints Row: The Third: You unlock them in story missions.
You can get 9 in all. You get the red light loft crib, the Saints Row mega condo, Two diffrent pent houses, two diffrent boat docks, You also get a airport hanger, and the Uneversity loft. Now the last one is the 3rd Street Saints hideout. Hope this helped -Nolan02 =)