Nevermind

The passwords are six letter long, but the middle two are the once that vary the most. The passwords take one of the forms: GZ..ZF, IZ..LK, RZ..BC, MZ..MM, AZ..HV, HZ..PI VZ..IN, PZ..WL depending on the level number.

Divide the level number by eight and take the remainder (0 to 7). Then the password is GZ..ZF to PZ..WL where the missing two letters are filled in as below. I don't think it actually matters which group of four letters you as a password, it still starts the game at the right level as long as the password has one of the above forms. The middle letters are based on the level number divided by 16 and the remainder. The numbers you get are encoded as follows:

0:M, 1:A, 2:H, 3:V, 4:P, 5:G, 6:I, 7:R, 8:W, 9:N, 10:Z,11:T, 12:L, 13:Q, 14:B, 15:F

The fourth letter of the password is the letter corresponding to the level number divided by 16 and the third letter is the remainder.

So, for level 18, remainder divided by 8 is 2, so password is RZ..BC, and dividing by 16 gives 1 remainder 2 so the middle letters are HA, and the password is RZHABC.There are a total of 255 levels.