How often can I get a free trial on ancestry?

Once..........however normally a couple of times a year they offer free access for a weekend/week........ and you can go to your local library and use the library addition every day for free if you wish to

probably just the one time

depends.  they might recognize you're using the same shipping address.  even if you put a different email.  then if you tell to ship it somewhere else and you'll pick it up, if you're doing it from the same location, they may recognize the same IP address.