Unfortunately, we don't have the report you're looking for. As Joe mentioned, we have a disc usage report that can be used to see the SIZE of your domains. I see the validity in your request though. I can understand how it would be tedious to click through each domain to see the user count. I'll change this thread to an Idea so that other users can vote it up. I'm going to modify your title of "How can I get a report that lists the number of email accounts per domain?" to something that indicates this is a request and not a question. In the meantime, I just want to let you know that you don't have to edit the domain to see the user count. If you click on a domain, the user count will be shown in the content pane to the right. You'll see Users under the Statistics section. It'll show you the number of users configured out of the number of users allowed for the domain.
Also, in case you weren't aware of this, you can set a limit on the number of users that your domains can configure. You can use domain propagation to propagate a limit across all domains or limit domains individually. To limit domains individually, right click on the domain and click Edit. On the Limits tab, change the Users limit to whatever you want. And here are the instructions on how you can use Domain Defaults
and Domain Propagation
I hope this helps! Let me know if you have any questions.