CASE statement

Greetings All,

I'm trying to use a formula text field to show a different text string based on the reference value in a multiple choice field that has 11 choices, i.e. when the answer in the multiple choice field is "A", show "AA" text in the formula text field. So in my formula, I am using CASE to test as follows:
CASE ([Field], "A", "AA","B","BB","C","CC" for the 11 values.

What I'm finding is that it only allows me to test against 8 values. For the 9th value I get a Boolean error message. This limit has been the same whether I'm using CASE or IF statements.

Does anyone have a suggestion on how I can go beyond the 8 value limit? Thanks for any light you can shed. I'm not a formula expert so I'm scratching my head on this.