How is the opportunity cost rate used in discounted cash flow analysis and where

How is the opportunity cost rate used in discounted cash flow analysis and where is it shown on a time line? Is the opportunity rate a single number that is used to evaluate all potential investments?If a firms earnings per share grew from $1 to $2 over a ten year period would the total growth be 100% but the annual growth rate less than 10%? please explain.Say my parents will retire in 18 years and they currently have $250000 and they think they will need one million at retirement. What annual interest rate must they earn to reach their goal assuming they dont save any addition funds?