Sources suggest that the 1.5L engine in your 2005 SCION xA takes 3.9 quarts with filter.
Before adding oil to your xA, make sure that your engine was cold and that your car was parked on a level surface when you checked the oil level otherwise you won't be able to tell if your xA has a low oil level. For most Scions, if your oil level is at the minimum mark on your dipstick, you should add about a quart of oil.
If you are unsure how much oil to add to your xA, add a little bit at a time and check your dipstick frequently. As per Carcarekiosk.com, adding too much oil to your xA can be harmful, so it is easiest to add a little at a time.
One of the most critical maintenance chores for cars is changing the oil regularly. Learn how to change your own oil quickly and without getting too dirty at Familyhandyman.com.