I cannot find anything which states the number of times a day a witch can use a generic hex in the witch section. Is the use per day of any given hex level related? Is it intelligence related? Is it limited only by available legal targets (some hexes do after all have built-in restrictions on the number of times that they can be used on a target)? Some clarification in the Witch entry on this topic would have been useful.
I think that you found your answer. They aren't limited except as written in the text.
If there IS no such limitation in the text, one needs to be errata'd in. Otherwise you end up with the RAW ability of a single 1st level witch to use their healing hex on thousands of wounded soldiers in a single day.
(3900 in an 8-hour shift, assuming they bring the wounded to the witch, an hour for lunch, and two 15-minute breaks.)