How often does your newborn eat & sleep?

My son was born April 21st at 9:25am and due to complications I didn’t get to try breastfeeding for the first time until around 8pm.

At 5 days old, he was being checked by nurses and they say he was dehydrated and not getting enough milk. (When he peed there would be the orange crystals). He also didn’t poop until being taken to the hospital on day 5 (his first poop was done inside me while I was pushing).

I’m still trying to learn how to tell if he’s eating enough & if I’m feeding him enough. As a first time mom I feel extremely guilty for not knowing that he wasn’t getting enough milk from me. Any tips or tricks to know when baby is getting enough or when he is actually full? A lot of times he stops eating, then 5-10 mins later he shows hunger cues again.