Answer:
204,826,993,236,684 possible variables
Explanation:
Upper case characters = 26
Lower case = 26
Digit = 10
Underscore = 1
Total Characters = 26 + 26 + 10 + 1 = 63
The following is the possible string selection of string of 8 characters that follow the above rule
First character = Upper case or Lower case = 26 + 26 = 52
Since there's no restriction on repitition of characters,
The second, third, fourth, fifth, sixth, seventh and eighth character could be any character
Second character = 63
Third character = 63
Fourth = 63
Firth = 63
Sixth = 63
Seventh = 63
Eight = 63
Total number of characters = 52 * 63 * 63 * 63 * 63 * 63 * 63 * 63
Total = 52 * 63^7
Total = 204,826,993,236,684 possible variables