In SingleThreadModel there are multiple thread and multiple instance for one servlet but at a time onluy one thread is running
Not necessarily. As David pointed out above, there are other ways to implement STM, like serializing requests. In that case, only a single instance would be necessary.
While in MultiThreadModel there are only one instance per servlet and there are many thread for each of request running simultenously.
Correct. (Note that there may still be multiple instances of a particular
servlet class if the servlet is configured multiple times in the web.xml file.)