Hi @david290. There are varied responses online, in the varied Microsoft forums, for this. However, Microsoft support states "There is a limit for non-Microsoft Sync Technology based accounts—you can add 15 mail apps to the app and unlimited drive (storage) apps (OneDrive, Box, etc.)." You can check their support page. to see for yourself as well. I know it may say Android and/or iOS, but it seems to function similarly for the new Outlook.
As for Microsoft's own email accounts, Microsoft isn't as forthcoming with limitations. I checked their Getting started with the new Outlook for Windows and didn't see any mention of limitations for Microsoft email accounts.
There seem to have been others, during the early stage, to have found workarounds. I have not personally tried those registry workarounds, so cannot speak to them. Mentioning them as it looks like it may have worked for some. You can check this Microsoft answers forum for more.
Hope this helps.
