Generally speaking, when you conclude students they still have access to the course material (unless restrictions are set up in settings). So, if you're looking to allow your students to have access to the content for future purposes, concluding while allowing them to still view the course would be ideal.
Deactivating technically still keeps them on the roster on the People tab in your course, but it prohibits them from accessing the course altogether. You can go back in and reactivate the enrollment at any time without having to re-add the student.
When it comes to the number of seats... I think it depends on the actual number of accounts associated with your institution's Canvas instance and not a particular course... but I could be incorrect about that. That would be something for your institutions' admin to reach out to your CSM about, just to be sure!