Just to echo what the other moms have said and add a few cents:
1. Try to avoid supplementing with formula. It will cause your milk supply to drop, since it's a supply and demand system. If you substitute some of the milk from your breasts with formula, then your body will think it doesn't need to make as much milk. Do this enough times and you definitely won't have enough milk and will have to supplement! Not a road you want to go down if you'd like to continue to successfully breastfeed.
2. As long as your baby is having 6-8 wet diapers a day, she's adequately hydrated. How much you can pump is not a good indication of how much milk your baby is able to get. Here's a link to a page that tells you what to look for to be sure she's getting enough:
http://www.llli.org/FAQ/enough.html
3. One more link - this is an blog with excellent suggestions and information about what you can do if your baby starts to stall out on the growth charts:
http://mostgladly.typepad.com/cj/2007/09/its-my-party-an....
4. Keep in mind that as your baby gets older she gets more efficient at getting milk out and your body gets better at making it. Unless she's going down for a nap or bed, my six month old usually only nurses for about 4 minutes on one side about every two hours. When she goes to sleep, she nurses for about ten minutes total (five on each side.) And believe me, she's getting PLENTY of milk - she's been exclusively breast fed until yesterday, when we started solids, and she is a fat little thing - she weighs 23 lbs!
So as long as she's peeing regularly and gaining weight (even if it's not a huge amount!) and is active and healthy, don't worry about how much you can pump out or her brief snacks.
Good luck!