When importing I would recommend turning off email for your users entirely. You can’t be sure that the import is going to be successful, and your users don’t need to know about the instance until everything is polished and ready to roll.
You can either configure disable emails to non-staff, or temporarily swap your mailgun settings for a copy of mailhog. Mailhog can simply intercept all outbound email, letting you see what will be sent without actually sending it.
In terms of email volumes, the activity summary is only daily for the first 50 registered users while bootstrap mode enabled, normally it’s weekly. You can disable bootstrap mode via /admin and AFAIK it won’t apply to imports anyhow.