You can put as many sub forms on the screen as you want.
However, there is a big difference between simply placing a whole bunch of
sub-forms on a form, and placing sub-forms INSIDE of sub-forms (that is
called nesting, and in access 97, you can only go 3 deep, and in a2000 and
later you allowed 10 deep. however, anymore more then 1 deep is very
confusing to the user). So, while I don't nest sub-forms very much, I
certainly do place more then one sub-form on the screen Just like you place
combo boxes on the screen, or text boxes on the screen, you can do the same
with sub-forms. After all, sub -form is really only a control