Thank you for your post.
Yes, you could also use units on the LPT ports - as you say, a maximum of 3. Therefore in theory you could have 7 ADC-22 units connected to the computer, however I have never tried this and would not know how efficiently it would run.
I believe the limitation is due to the driver however I would need to check this to make sure. I am not aware of plans to increase this limit.
Hope this helps