Breastfeeding

I have 4 kids ages 8💙,5💜,2💙 and 3months💜

I have breastfeed all my previous kids till 3 months and usually I can’t wait to be done my now with my last baby I don’t want to stop i want to continue breastfeeding and use formula when im away at work which is only. 3/12 night shift and she sleeps through the night but anyways i feel like my milk is drying how can i tell if she’s getting what she needs? Yesterday and today i feel like she’s not getting full and usually just goes to sleep because she gets tired of sucking idk baby im just scare shes not eating I haven’t gave her any formula since im waiting when i start working on the second week of July and i also have 20+ bags of breast milk