This question is second only to when should I get my maternity photos? Ideally you want to get them in the first week to 10 days max. They grow so fast the first month. You want to capture just how tiny they were when they first arrived.
Newborns are always a challenge as they tend to live on their own schedules and moods. So I always allow plenty of time. No need to rush. Plenty of time for feeding and diaper breaks. And sometimes they just need to be held a bit to be comforted before the next shot.
Many parents decide to do an album to document the first year. Starting with the newborn photos and then every 3 months up to the first birthday. It becomes a keepsake that will be treasured for a lifetime.
Whatever you decide to do, the sooner the better for your newborn session. Like I mentioned before, ideally 10 days old is the maximum age for their newborn photo session.