Hide the attendees in appointments like bcc does and performance implications if sent to million users

Hi,

I have a requirement of creating appointment with response requested flag for multiple users. User count may be sometimes in millions.

My question is:

1. What is the limit of recipients count? Is there any performance implications?
2. The recipients should not be provided with other recipients details like BCC functionality. Is there a way to do so?

I have an alternative way to achieve same: Sending individual appointments. And each appointment will have extended metadata to identify the appointments are for one particular purpose. What is the impact of going ahead with this approach in terms of performance?

Thanks,