A good start is to position a room-size air cleaner in the room where you spend the most time in a 24 hour day and that is usually the bedroom. Take a measure of your bedroom and shop for a unit designed for that size of room to ensure adequate efficiency.
Should your particular needs demand more allergen control, you may want to consider a whole house unit and placement will depend on the manufacturer's recommendations for that model. You can check the product manual for those details.
If you decide to add another unit to your home, I would position it in the living or family room so several family members can benefit from its operation. You could also add an air cleaner to a home office if you spend a lot of time in that room.
Always position an air cleaner as per manufacturer's recommendations as some units need distance from walls or furnishings for best operation.
Before you start shopping for an air cleaner, review my buying tips in Before You Buy an Air Cleaner