Hi Florian!
Anastasia here, an ex-Braze lifecycle marketing consultant. Hope your day is going well!
Creating a segment composed of all people who received the relevant message, last engaged with a message at that time, AND whose subscription status is now unsubscribed should give you a strong sense of how many recipients unsubscribed in that particular message.
To get more granular, though, I'd recommend one of two options:
1. Partner with your BI team to run a Currents export of all users who had an Email Unsubscribe event on their profile associated with that message ID. This option would require you have access to Currents.
Docs: https://www.braze.com/docs/user_guide/data_and_analytics/braze_currents/event_glossary/message_engagement_events#email-unsubscribe-events
https://www.braze.com/docs/user_guide/data_and_analytics/braze_currents
2. Or, for future messages, use link aliasing to add an alias to your unsub link. Then, you can create a segment of users who click that alias.
Docs: https://www.braze.com/docs/user_guide/message_building_by_channel/email/templates/link_aliasing
Hope this helps!