No need to tag us buddy, We get notifications on new posts
I guess the approach to take would depend on your workflow, There is no reason I can think of for one workflow not to be able to run 10,000 times or process 10,000 records at once.
Is this 10k emails coming in at once, 10k emails being sent at once or just 10k users being able to send an email from n8n?
So 10k being sent at the same time should be fine but it will come down to how you set up n8n and you will need to have the resources to deal with that.
I would probably start by doing a test with what you have then scaling out from there, Sadly there won’t be a magic number for it and it will require some manual testing.