You can download a current attendance report during the webinar as well as a final report afterwards. The final report includes info such as the percentage of registrants who attended and average attendance time.
During the webinar
If the participants pane isn't open, select Show participants in your meeting controls.
Select the down arrow next to People to download the current attendance report.
You'll find the report in your device's Downloads folder.
Note: For large webinars, the report will include only a partial list of attendees.
After the webinar
In your calendar, double-click the webinar event and select the Attendance tab. You can view the data directly in Teams.
To download the report, select the Details tab and then the Attendance button.