Check your ASP.NET
<processModel \> section for the framework. c:\windows\microsoft.net\framework(64)(version)\config\web.config and machine.config:
MSDN: <processModel> Element
That has some limits on the request queues and threads.
However, the bigger issue is why you have 1000 concurrent connections. If you can get users of the server quicker then it won't build that high. If you can find a couple problem pages and speed up performance then the server will be able to keep up and the 1000 limit will be a non-issue. Generally speaking, 1000 concurrent connection is a LOT of traffic and each web server doesn't normally intend to have to handle that much.