Hello Brent,
First of all, on behalf of the Disney Parks Moms Panel, I would like to congratulate you on the upcoming birth of your baby. What exciting news to be adding to your family. My family is a party of five, but if we could have, we would have had more.
Based on my personal experience, our baby did not count in our numbers until he was 3. We stayed many times in a standard room that was meant for up to 4 people and we had our youngest son with us, making five. We often asked for a pack 'n' play and there was never an issue. So, I would think you could reserve a room for up to five guests and still have plenty of room for the newborn. Children under 3 years of age are not typically counted as they are considered "free" for admission, dining, etc. Again, this is based on my personal experience.
Speaking of babies, I also wanted you to be aware of the amazing
Baby Care Centers that are available in all four theme parks. These are a wonderful resource and place of quiet escape when
traveling with newborns and little ones.
I hope this helps with your planning!
~Allison L