Number of possible an md5 string:
16 possibilities for each digit (0-9 (10 possibilities) and A-F (6 possibilities)), there are a total of 32 digits so the number of possible combinations is : 16 ^ 32 : 3.4028237 * 10^38
Another way you could calculate it is:
Each Digit of an MD5 represent 4 bits (Hexadecimal). So for 32 digits we have a total of 32 * 4 = 128 bits so 2^128 = 3.4028237 * 10^38