Looks like we have a mismatch between our frontend and our backend! Our backend is explicitly only allowing a single email on that free email node, while the frontend is allowing as many as you want. We have discussed internally, and we are going to “split the difference” - and allow the email node to send an email to up to 5 recipients. This change is a little involved, so we won’t be hotfixing it, but we should be able to include it in the next release (which is planned for later next week).
In the meantime, the more advanced Sendgrid node will work for as many recipients as you want - and Sendgrid has a small free plan you can sign up for.
Thanks for the bug report, and we will let you know when the fix is in production!