Which ball number is in the middle of a game of pool if placed smallest ball to largest?

This is not possible in any common game of pool. Only Straight Pool allows the balls to be placed in nearly any position to accomplish this - no other common pool game allows placing the balls in that order. The middle of the rack is in between the second and third row of balls. It appears what is being asked is what ball will be in the 5th ball position, with one ball to left and one ball to the right if all the balls are arranged lowest to highest. This would be the 5 ball. However, Straight Pool requires that the 1 ball and the 5 ball be placed in the corners.

Therefore, no pool game allows the balls to be placed in the way required - so, any ball may be in the "middle" position depending upon the game of pool being played.