Breastmilk & Formula

Hey mamas. What kind of formula is the best formula and do you mix it in with your breastmilk or do you do two separate bottles? I’m out 12 weeks and I have 6 more to go before I’m back and work and baby is at daycare. I’m trying to prepare myself and her to have enough milk. I have solely breastfed since I’ve had her. However, I would like to introduce formula because I’ll be super busy. A lot of other Army mamas just do all formula but I would like to keep breast milk in the mix. I pump and have began freezing but i know it won’t be enough. Help me out 💗