It appears that the cap is 100 users per email. Is this correct?
Yes, because the Users list paginates at 100 you can only send 100 emails at a time. You can send emails page-per-page if you have more than 100 users.
Or, for a more robust solution, I'd recommend integrating your site with an email marketing tool like MailChimp or ConvertKit.
Happy to help if you have any further questions at all.