Child domain setup

Hi All,

I am planning a setup.

Where there are 7 Child domain under a parent domain on Win Srv 2012R2.

All 7 Child domains corresponds to different Companies of one group at different physical locations.

Please provide me the information based on below mentioned points keeping in view that there should not be any authentication issues when the link is down between any of the domain.

1. How many Primary Domain COntrollers and Additional domain controllers should be planned as per the best practices.

2. All the child domain has domain controller

3. How to design the Entire hierarchy for Root domain and 7 Child domains.

Thanks in advance

K2