When you try to subscribe for alerts to specific user account in Sharepoint and get the following error:
You have created the maximum number of alerts allowed for this site
it may be so that you have reached alerts limit for this user in current site. This per-web application setting and it is possible to extend this limit in Central administration > Manage web applications > web app > General settings > Alerts:
By default it is set to 500 alerts per user. After you will increase this limit error should disappear.