You will be better off asking this question in a more appropriate forum. If the question is specifically about Tomcat then the Apache Tomcat forum should help:
https://coderanch.com/forums/f-56/Tomcat As with lots of these things, the number of processes, and threads supported is configurable, and each implementation will have it's own foibles over when new processes are created.
There is a subtle distinction between threads from the httpd perspective (i.e. to handle requests for anything including images / static html), and threads allocated to servlets.
If the number of threads a process is alloated becomes too large then processes spend too much time (and memory) handling threads rather than dealing with the task in hand. Whether that is for a httpd process, or a process that the servlet container runs in.
Rufus.