Advisory Opinion Letters aka Reccomendation Letters are usually included in an application. These are most often drafted by your lawyer or you and then sent to the person to review, print, and sign on their letterhead.
USCIS does not have a minimum or maximum number of letters they ask for so it is really up to you. Most people submit anywhere from 5 to 12, the average being somewhere on the higher side like 9-10. The letters should come from experts in your field, people who have worked with you and can vouch for you. A tip in this part of the process is to also include press about them so as to prove their credentials and avoid multiple letters from your academic instituion.