If someone say(verbally) his/her password is 12345678. Some example possible password are
"12345678" or "Onetwothreefourfivesixseveneight" Or "1twothreefourfivesixseveneight", "1twothreefourfivesixseven8" ...
Letters can be upper and lower case both. So what is the number of total possible passwords or what can be upper bound for the number of such passwords.
Some smaller cases
If they say it's "1" : 9
Then we have One, ONe, ONE, ... 8 possibilities and 1
so there are 9 passwords.
If they say it's "12": 82
then we have 8*8 only for digits case.. =64
then 12 = 1
mixed are : one2, One2 ... 8 possibilities. + 1two, 1Two .8 possibilities = 16 should be 64+2+16 = 82
We can discount far-fetched interpretations such as "444" for "three four", since this is ungrammatical.
For each digit, you have either the digit itself, or the corresponding word. Each letter of the word can be upper or lower case, so in total there are $1 + 2^l$ possibilities for a given digit, where $l$ is the number of letters in the word.
Thus, the number of possibilities for $123456789$ is $$(1+2^3)(1+2^3)(1+2^5)\cdots(1+2^4) = 128,711,132,649.$$
I have discounted far-fetched interpretations such as "444" for "three four", since this is ungrammatical. The one exception would be "2" for "one two", as the singular here is grammatical.