how many adresses can I have in a single Thunderbird email?

There's no practical limit in Thunderbird... but there probably is with your email provider. For example, Gmail has this to say, https://support.google.com/mail/answer/22839 "You may see this message if you send an email to a total of more than 500 recipients in a single email and or more than 500 emails sent in a day. When you get this error, you should be able to send emails again within 1 to 24 hours." If you do decide to send such a mass mailing you would be best off using Thunderbird's Mailing List feature. https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird