I have to credit supplementing with why I was able to breastfeed my daughter until 18 months - when she pretty much weaned herself. With my son it was a nightmare and I had only barely gotten my milk in before I gave up in frustration. My son seemed happier on formula when we tested it so I switched him. With my daughter, I was already having trouble getting her to latch on at the hospital and the nurse told me to supplement if I felt she wasn't staying on long enough. That saved it for me, and while supplementing isn't for everyone - anyone who is worried they aren't able to keep up with the demand should consider it.
I generally only gave my daughter 2 oz - 4 oz formula bottles, and only AFTER I'd already let her have whatever I had available for her. I did continue pumping sometimes to try and increase my supply. Babies go through growth spurts where they need more. While the mom's body catches on to this - I think sometimes it can take a little longer than the baby likes to wait! I found that sometimes I DID make enough for her and other times it was lacking until I caught up. I think we may have actually gone through one span of two to three weeks straight where I didn't supplement at all, but that was also after solid food was introduced. Technically, that's like supplementing too - once the baby is old enough, of course - and that will ease the formula woes considerably. She still wanted more after that nighttime feeding though - it tended to settle her in for a good night's sleep to have some snuggle time with mommy's milk first and then follow it up with a couple ounces of daddy's bottle.
We also tried, especially in the beginning, to mostly let her get bottles from daddy or someone else and rarely ever from me. The amount I gave her slowly increased, but I always gave her what I had available on both sides BEFORE letting her have anything else. And the slow-flow nipples on the bottles are important. It takes some work for a baby to get to the breastmilk - bottles are easier. If they get it too fast, they gag/choke from it and if they decide it's easier from a bottle - some babies will begin refusing the breast because the bottle is easier for them to get the milk from. I did not seem to have this problem with my daughter though - I think she knew that I coveted the special snuggling we were doing and enjoyed it too. And I know my husband enjoyed being able to feed her the formula too - he was wonderful about helping me through my own frustrations about not making "enough" for my kids all the time.
Good luck!