Trying to use a 7 segment display to count 0/9 with a potentiometer
You miss the break statement at the end of each case. e.g.:

  digitalWrite (SegmentF, LOW);
  digitalWrite (SegmentG, HIGH);

  delay(1000);

  break;

case 4:


This causes each case to perform, and continue with the next case. However, since there is a delay of 1 second, it looks like it is counting.

Actually, I would expect it counts from a certain value depending on the pot meter to 7, since there is no digit 9 implemented, and the delay for the 8 digit is missing too.

To further improve your code, you can add the delay not in every case but at the end, since I assume there always will be one digit selected.

Also you can make a function void showDigit with a parameter for each of the possible segment states and replace all (almost) duplicated code by that function call, or send the digit and use showDigit to set internally the states of each segment (by using a boolean array[nr_of_digits][nr_of_segments], somewhat like Nic Hartley proposes below.