Hold each value in an array. Declare an array of size 20. Then using a loop structure, accept the user inputs into the array, sort of like this:
for (i=0; i<20; i++)
cin >> itemValue[i];
You'll have to set up some method to bail out of the loop early. Like if the user enters '*' instead of a number. Save the value of i at the time of exit so you'll know how many numbers were entered. Then, have another loop to go through your array from 0..i, cout the value, and figure out your min, max, avg and sum.