What might be the problem with Site Users web part in SharePoint Online. There are plenty of inlogged users but the web part returns only one user.

Hi,

As far as I know, by default, the Site Users Web Part will only show all the users and groups with direct permissions on this site. Any user in a group will not show up there directly. You might want to have a check on this aspect, change the Display Type in Web Part setting to show users in a specific group.

Also, here in this forum we only focuses on questions and feedback for Microsoft Office client. There is not so much about SharePoint Online here, for further assistance, I would suggest you post in the dedicated forum of SharePoint Online:

https://social.technet.microsoft.com/Forums/msonline/en-US/home?forum=onlineservicessharepoint

The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thank you for your understanding.

Regards,

Ethan Hua
TechNet Community Support