There is no real hard-and-fast rule about saving for real passengers, but SWA flyers have a tendency to create their own etiquette when it comes to the seating process. Generally speaking, saving within your row is acceptable as long as you realize that there is no "right" to save; if another passenger really wants the seat, you are expected to graciously give it up.
For each individual with an A pass, two extra seats is the understood limit. It is always best to say who you are saving for, so that it is clear that you are not trying to hold open a seat for a "phantom" passenger in order to get more space for yourself. Saving for "phantom" passengers is EVIL, and if there is a passenger on the aircraft who is counted for that seat, the saver will get busted eventually.
Do NOT attempt to seat-save in the exit row; that is also evil, and won't work, anyway. Generosity is in short supply when it comes to exit row seats.
The proper way to do it is for the person saving to sit in the row's aisle seat, and if someone hesitates and gestures at the seats, politely say something like, "I'm hoping to save these for my wife and child." It is more polite to seat-save at the rear of the cabin rather than the front, and you are much more likely to be challenged for seats in the first few rows. BTW, don't spread belongings over the seats in order to make it clear that they are saved; that will make other passengers and the FA think you are saving for a phantom, and you will be asked to remove those items.