Regarding, How to configure thread pool via CLI mode? and also what is the use of thread pool?
It really depends on which thread pool you actually want to configure? For example if you want to configure the EJB thread pool then you can refer to the following article.
Similarly you can configure the http thread pools as well as described in :
Regarding your query, Where you mentioned the issue with issue with connection pool. Can you please clarify on which connection pool is it ? Is it DataSource pool size? or EJB pool? etc..
Also you mentioned that the "application doing down" Can you please clarify what do you mean by down? Did your JBoss suddenly got killed, Stopped, Crashed ?