Do dentists or orthodontists usually order you retainer the exact number as last Invisalign instead of scanning teeth?

You are correct that the best fitting retainers are made from new scans or impressions after the composite attachments are removed and teeth are polished. This may not be the standard for the office that you are in. If they offer lower fees below the market this may be an area where they can save some of those higher fees as well passing the savings on to their patients. If your case tracked well to the end this may not be too much of a drawback. Good luck to you.

It is normal to remove invisalign attachments on your teeth after the last aligner has been used. If all is well with the movement and your case has tracked well with each aligner then you can use the last aligner as your model for your retainers. You don't necessarily need to be rescanned. I would recommend that you continue to use your last aligner until the retainers are delivered. Also, ask your dentist about your concerns.