8
Unless you have persistent connections enabled, the connection gets closed automatically.
A "question" is a query. So that error means you're getting 50000 database queries within the specified time period.
The only way I know to troubleshoot it is to add some code to log all the queries, along with some identifying information (e.g., timestamp, IP address, user agent). Then you would be able to tell whether some particular visitor, such as a robot, is causing the problem, or whether it's just that your site is very busy due to normal traffic.
The problem might be lessened by enabling caching for, or not using, various blocks or modules.