The probability will depend on what we regard as being characters.
Will letters, digits and symbols (such as ?, ! @, *,~,etc) be included?
Let's consider just letters of the alphabet. There are 26 letters, but as each can be upper case or lower case, we have 52 to choose from.
There are no constraints on repeating the letters, so for EACH letter of the password there are
The number of possible passwords :