Why did my email campaign go to far fewer people than I have customers?

Campaigns only go to your newsletter subscribers whose status is Subscribed, not to every customer. Customers who never ticked a newsletter sign-up are not emailed, and nor is anyone who unsubscribed.

To check:

1. Go to Marketing → Newsletter Subscribers. The Subscribed count is the most a campaign can reach.
2. In the campaign, check Audience. Subscribers who are customers or a group option narrows it further.

To grow the list, add a newsletter sign-up block to your storefront, or add people who agreed to hear from you with Add a subscriber.