Forcing items to stack?

Hey, I'm very new to coding with C#. And I have so far made a plugin that makes furnaces generate Pookies. However there is a issue. Whenever it generates more pookies it will put it into the second slot. I can't find any documentation or tips on how to make the new pookies go into the old stack. So it goes like this

5 Pookies = 10 pookies in 1 stack. But no it goes

5 Pookies, 5 Pookies

Any help is appreciated!

 

Cause pookies dont stack on vanilla maybe, Try to search in the code of stack plugins?
I have no idea what im talking about just thinking along with u

 

Thats what im doing. Just quite confusing considering I started c# today.