The tactic that some students use, and I am not overly fond of, is the "submit with 1 school" method. Here you prepare AMCAS as usual, but instead of all your schools, you list a single school. This allows your application to be processed and verified. When you get your MCAT score back, you can quickly add more schools and AMCAS will transmit a previously verified application in one business day. Which "1" school you should pick is a matter of opinion. You can pick a "throwaway" school, such as an out of state with low OOS rates. Or you could be pick a "high reach" school, one that you might have an outside chance with if you got a super MCAT score. In either case, you do not have to fill out the secondary (and pay the secondary fee) for this school, thus your application will effectively die. You should at some point in the cycle inform the "throwaway" or "high reach" school you are withdrawing your application.
I am not fond of this method as you should be spending time prepping for the all-important MCAT and not preparing a full AMCAS primary, which takes as much time for 1 school as it does for 25. So if you are in need of a improved MCAT and are taking one later in the cycle (from June on), you should prep for the exam and once you have taken it, but before the score in returned, you then can work on AMCAS and pre-write secondaries.
Lastly, what do you do if get a bad score on the MCAT retake. Applicants seriously need to consider not applying to any additional schools and skipping the cycle. Applying with a poor retake, especially one lower than your original score is likely lead you to being rejected and thus being in a weak position next cycle as a reapplicant next cycle.
Applying without knowing all your MCAT scores is a mistake that all applicants should avoid