The http://nationbuilder.com/pricing page makes a distinction between the total people and the emailable people you can have in your database at specific pricing tiers. What is considered an "emailable" person? Does everyone in my database who has an email count towards my "emailable" maximum?
An emailable person is someone who is considered to be opted in to your communication. You can filter for "emailable" in your People database to find the number of profiles you have in this category. You can also go to Settings > Billing > Plan to see this number, and how many more you can have to reach the maximum of your current pricing tier.
The emailable category excludes contacts who have unsubscribed or have invalid email addresses. Note that a profile will still be considered emailable if one address in the profile is bad, but others are valid.
If you have exceeded the number of emailable profiles you have in your desired pricing tier, you can batch unsubscribe a segment of your contacts to to lower your monthly billing. To do this, add your emailable profiles to a list and batch update them to change their "Receive Email" status to "No." This will opt-them out of your future blasts, and remove them from your "emailable" count.
If you're not sure which contacts to remove from your email program, you might want to consider running an opt-in campaign. Alternatively, you can use the "Opened email blasts" or "Clicked email blasts" filters to see who has not been engaging with your emails recently. As a bonus - this will also improve your email deliverability!
Complete and current information on how to use NationBuilder is available in the documentation section.