When I was breastfeeding, often at the end of the day my milk wasn't as much if I'd had a busy day, hadn't eaten enough, or was tired/stressed (sounds like the life of a mum of a newborn huh!). So I tried to drink lots of water, eat lots of good healthy food, and also had things like almonds, bananas and milk to increase my milk supply. All those things helped lots - as well as making sure I rested throughout the day.
Just be aware that some mums who do formula top-ups may find that as a result their milk decreases (it's a demand and supply situation, the more he feeds the more milk you'll make) or that baby prefers the bottle cos he gets milk quicker. Just a couple of things to consider as you make your decision