Multiple domain controllers

Need some guidance on how to plan for multiple domain controllers so it can scale.

Our company has sites in multiple US states and the plan is to have a domain controller in each state.

My manager mentioned a single root domain and multiple child domains for each state but I've read that's not a good choice and can be difficult to support.

I believe the way is to simply have OUs for each domain controller but I'm not sure of the limitations for that route.

Are we stuck with creating a separate forest per each domain controller?

Microsoft recommends no more than 10 domains per forest but not sure if that applies to us since we'll only have a few hundred objects per domain controller.