On the dashboard, you will see the number of Total Subscribers you can send email to:
There are a few reasons why you might be unable to send email campaigns to certain contacts:
They've unsubscribed from your list. In any email that you send to your list, an unsubscribe link is included in the footer. If a user clicks unsubscribe in one of your emails, they'll be unsubscribed from your list going forward, preventing you from emailing them unless they resubscribe.
They've marked your email as spam: If you email a user and they mark your email as spam, they're unsubscribed from your list going forward. This is to protect the deliverability of your future email campaigns. (It hurts your deliverability score every time someone marks a message as spam, and these contacts are particularly likely to mark your message as spam again.)
They have an invalid email address: When you import or collect an email address through one of Hive's Capture Apps, those email addresses are validated to make sure they're deliverable. If an email address is old, inaccurate, or invalid for another reason, the contact is marked as invalid within Hive.