I think "en tå for mye" is correct. "En tå for mange" sounds strange to me. I general we use "mange" when the noun is plural, "mye" if it's singular. But in the case of having twelve toes, I would still say "to tær for mye".
I shared this sentence with a native Norwegian speaker, and he also agreed that it should be "én tå for mye".
I'm curious to see a moderator response on this. I wonder if it's a regional difference or something. I'm only learning so i have no claim or position.