How many characters can you enter for duty MOS?

In the US Army, for enlisted personnel, all MOS have a total of nine characters. The first two digits are the Career Management Field, and are numbers, while the third is a letter which pinpoints your occupation more specifically - these three characters comprise your actual MOS.

The fourth character is a number, which is a Skill Level Identifier - 0 for a entry level trainee, 1 for Private through Specialist, 2 for Corporal (DA approved) and Sergeant, 3 for Staff Sergeant, 4 for Sergeant First Class, and 5 for Master Sergeant through Sergeant Major of the Army.

The fifth character is a Special Qualification Identifier, which is for any specialised training and skills you've achieved. For example, being parachute qualified, I had a "P" as my SQI. If you have no such identifiers, you're given a default "O" (the letter.. SQIs are always a letter).

The sixth and seventh characters are for Additional Skill Identifiers, which may be a combination of a number and a letter, or may be two additional skills each assigned an individual letter. If you have none, you get the default "OO".

Eighth and ninth characters are Language Identification Codes. If you do not speak a foreign language, you are again assigned "OO".