whisk 2 cups milk and pudding mix in a medium bowl. stir in 3/4 cup of cool whip.
to make the parfaits, it does not really matter how you layer it. the number of banana slices and vanilla wafers you use will depend on the dish you use and the look you want to achieve.
my preference is to crumble 2 vanilla wafers into thh bottom of the dish, layer with 3 banana slices, then 1/4 cup of pudding. for the next layer, break 3 cookies in fourths or halves and place them on top of the pudding, layer with 3 banana slices and then add 1/3 cup of pudding and finish with a dollop of cool whip. repeat, chill and serve.