I'm interested in the answer to this as well. I'm in the process of switching to FormMail from another script. I have a form that has 51 fields, 37 of which are required.
So far, the only limitation that I seem to have found is the size of this field:
<input type="hidden" name="required" value=""> Somewhere around the 34th required field, the error messages start breaking down (even if the user has filled in the first 33 correctly, the field name gets truncated, so the user doesn't know which field they forgot). I'm using fmbadhandler.php and a custom error page.