look the equation you use is not balanced instead, there should be 2 beside the propanoic acid, which means the number of moles consumed would always be twice the number of magnesium , so he calculated the number of mg moles (0.25) and multiplied it by two to get the number of propanoic acid moles that can be consumed but i don't know why he minuses and adds to "one" i mean what makes him think that the number of moles in propanioc acid is one to minus from you said you only wanted to know why he multiples by two so i hope that answered your question good luck
if you know why he adds and minuses from one please tell me if you have time
