I assume you have some ammo for your first range trip. You've thought about it, and think you know how much ammo to take with you. May I offer some advice?
-----> It isn't enough. <-----
Whatever amount you think you'll shoot for that first time outing, it ain't near enough. Every one of us who has ever bought an AR-15 and taken it out for that initial shoot - and I mean every single one of us - wishes we'd brought two to three times the ammo we did. One of the amazing things about shooting an AR-15 is how quickly it goes through your ammo supply. You'll have this big ol' grin on your face and reach for the box or bag to load another magazine and go, "Oh man! How can I be outta ammo already?!"
Trust me on this. Go. Buy. More. Ammo.