In order to maintain a supply you should BF as much as possible, supplementing will only worsen your supply, there are many options to boosting your supply, but formula will decrease it. I pumped breast milk when I went out or had a babysitter, that's an option. Also, it may upset your little one's stomach switching on and off, some babies don't adapt well to the change off if it's frequently.
I'm just wondering just in case I can't supply enough I had that problem with my son because he ate a lot. Plus when I go back to work or if we go out and need a babysitter
You can, when I nursed my first and had problems, the hospital even provided formula as a supplemental feeding for her. Any amount of breast milk the baby receives is better than none at all. Good luck.
The danger of supplementing with formula is your milk supply will suffer. And to breastfeed well you need to exclusively breastfeed as much as possible especially at the start to help your milk co3 in and establish a good supply. Is there a reason you want to do formula ?