Enabling membership reminders - will it only mail expirations valid this day?
My advice would be to test this carefully in a test environment with the setting 'Outbound Email' set to 'redirect to database'. This means emails do not actually get sent, but you can see them in the database, and so also in the contact summary. That would be the best way to reassure you. Whatever someone says here about the core CiviCRM functionality, it will not show you what will happen in your specific installation and configuration. So I would always strongly recommend a local test.