how to determine what plays to call ?

I understand that much, so basically you have 10-20 plays scripted and you call them accordingly. my next thing is say its 2nd and 7 and tendency wise the defense you are facing knows that you are running a pass play do you stick with what you called or should I have a check with me system employed where we check from pass to run or vice versa? and how do I scheme a defense formation wise , what do I do to eat a defense alive because I was considering running between 6-8 different formations and about 25-40 plays being able to run them from a 1 to 3 back set or 1 to 5 WR sets?