No real break in or special maintenance other than what's in your manual. Or specific round count but you do want to make sure the gun is reliable before you bet your life on it. Test all ammo and magazines you plan to use. I'd sure want it to fire at least 200 rounds with no malfunctions.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek