That depends on why you are asking. You did not specify model year or type of transmission so I will assume that we are talking about a four speed automatic transmission. If you are draining the fluid by way of the drain plug on the left side of the transmission the amount is 4 quarts, give or take. Make sure that you are using the correct Transmission Fluid for your car by checking your owner's manual. Since about 2005 they use a type HP fluid. This stuff is not cheap, but I have heard Horror stories from people who have tried using Dexron.
The gas tank on the 2012 Subaru Outback holds 18.5 gal..
The gas tank on the 2002 Subaru Outback holds 16.9 gal..
The gas tank on the 2005 Subaru Outback holds 16.9 gal..
The gas tank on the 2004 Subaru Outback holds 16.9 gal..
The gas tank on the 2008 Subaru Outback holds 16.9 gal..
The gas tank on the 2007 Subaru Outback holds 15.9 gal..
The gas tank on the 2003 Subaru Outback holds 16.9 gal..
The gas tank on the 2009 Subaru Outback holds 16.9 gal..
The gas tank on the 2011 Subaru Outback holds 18.5 gal..
The gas tank on the 2014 Subaru Outback holds 18.5 gal..
The gas tank on the 2006 Subaru Outback holds 16.9 gal..
The gas tank on the 2013 Subaru Outback holds 18.5 gal..