Hi,
My LO was taking 5oz every 2-3 hours at twelve weeks and had between 5 - 6 bottles a day at about this time she started to sleep longer too and would go down at about 9.30 and sleep through to 6.30 - 7 ish. She is now 16 weeks and is now taking 6-7oz every 3 hours and a total of 5 bottles a day. She sleeps from about 8.30 to about 7. I think that between 20 - 30 oz in total per day is about right for a baby of 12 weeks.
Hope that helps a bit!