Hi there,
Do not wait until you receive your August MCAT score to complete the AMCAS application. Your score will not be back until around October 15th. If you wait that long you will be way behind.
I took MCAT (only once) in August but my AMCAS application was in on June 30th. I already had secondary applications from four of the six schools that I had applied to. When my MCAT scores came in, the rest of the secondaries came and my interviews came shortly after that.
Do realize that some schools will not send you secondaries until your MCAT scores are in especially those that screen for MCAT and GPA.
Get as much of your application done as quickly as possible.
njbmd 🙂