Buy as many smoke alarms as it takes to give your home complete coverage. You obviously increase your chances of survival with each alarm that you have, but one on each level of the house is the absolute minimum.
You should have a smoke alarm in each bedroom, in the hallway close to each sleeping area and in heavily occupied areas like the living room.
Smoke alarms should be placed on or near the ceiling where smoke tends to be heaviest.
Smoke alarms should not be placed directly over stoves, ovens or fireplaces.
When bedroom doors are left open, you should have at least one alarm in the hallway outside the bedroom area.
Smoke alarms don't last forever. Replace yours once every 10 years. If you can't remember how old your alarm is, then it is probably time for a new one.