If I had 30 groups and each had 8,000 subscribers, the Basic plan for 10,000 subscribers would be ok? But if just one of the 30 groups had 12,000 then I’d need to go up to the next level, right?

Yes, you are right. A “subscriber” means a unique person (email address) on your list. For example, if you have a list of 8,000 recipients, the Basic plan is enough. And you can send as many emails to those people per month as you want. The number of email messages (campaigns) per month does not matter.

The most important thing is to watch that the number of unique email addresses on your list does not exceed 10,000. When you see that you have more than 10,000 subscribers on one or several lists, you need to upgrade to a higher plan.