Why only 75 connection are achieved in web application?

I have been assigned a bug, assume x application is built on java , groovy and grails. The Bug is, when a .net application is trying to connect a x application it works until 75 connection. 76th connection fails, Connection pooling is implemented in x application.

more information: .net app uploads a document later it connects to x app which is a api stores in temp and forwards it to third partry app which actually stores it database.

Can anyone help me understand what is the issue and is it related to session pooling? if so can you please suggest me some materials to refer.