That’s nine :smack:
I’m blathering . It’s enough to give me nightmares. If you’ll excuse me, I have to use the chamberpot (I ate too many of mother’s infamously bad fruitcakes).
One common reason for wanting such a word is to code prices. f=1 u=2, etc. The above word was used for the wholesale price on tags at the pharmacy I worked at in high school. I’ve seen similar schemes used in car lots.
Of ten-letter words of that sort, there are far too many to mention them all. You can even make sentences of them: Hysterical blockheads presumably discourage bankruptcy.
The longest words of that type I know are ambidextrously (14 letters) and uncopyrightable (15 letters).
I should note that “ambidextrously” is the only one that doesn’t have a red squiggly line under it as I post this, though, so the rest aren’t particularly common.