Words Words Words

Back in graduate school, in the writing workshops, we were told that Tolstoy said you could use the word 'suddenly' just once in a 1200 page novel. I have no idea if this is true, but I passed the rule on in my workshops.

Working on Velocity 4, I'm wondering how often I can use the word 'dubiously.' As in, "Are you sure?" she asked dubiously.

What do y'all think?