Hi, 

We built a provider hosted app for performing a custom search based on certain criteria. Our environment is a medium sized farm with multiple w.f.e servers for both sharepoint and web apps hosting provider hosted apps. We were trying to perform load testing with a third party tool to determine the number of concurrent users sharepoint can service requests at a give point in time. 

Upon running the tool, SharePoint started throttling requests by throwing an error "Exception occurred. (429). Too Many requests. ...". Now, the error was occurring at the same place since there was only one call to SharePoint to execute a search query. The provider hosted app was written using .NET CSOM. 

We had performed the test with one user account and three user accounts. After 60 concurrent connections, exceptions such as "Too many requests" were thrown. Can someone shed light and interpret this issue? We are running SP in our local environment and not on office365.