Are there any letters/numbers that should be avoided in an ID?

Use uppercase letters and digits only, omitting I, O, 1 and 0. That should be unambiguous enough. This same issue comes up with car licence plates.

1. What does 8 12 characters mean in a password?
2. What is the A to Z alphabet?
3. How many possible ID number are there in which all 3 digits and 2 letters are different?
4. What is it called when you replace letters with numbers?

What does 8 12 characters mean in a password?

This means that your password must be of at least 8 alphanumeric character and can reach to maximum length of 12 alphanumeric characters. The term 'alphanumeric' means the combination of upper-case and lower-case alphabets or letters and numbers and special characters (such as @, #, $ etc.).

What is the A to Z alphabet?

The English Alphabet consists of 26 letters: A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z.

How many possible ID number are there in which all 3 digits and 2 letters are different?

The same applies for the three digits. So for a license plate which has 2 letters and 3 digits, there are: 26×26×10×10×10=676,000 possibilities.

What is it called when you replace letters with numbers?

Leet speak, also known as hackspeak or simply leet, is the substitution of a word's letters with numbers or special characters.