Servlets implementing SingleThreadModel cannot be run on two threads at the same time. So, if there are mutiple requests, the servlet container will create a pool of instances for this servlet and dispatch each request to a different instance.
Note that SingleThreadModel has been deprecated in Servlet Spec 2.4 and programmers are strictly advised not to use this. Though SingleThreadModel seems lucrative for solving mutil-threading problems, it actually does not !!