I had to games with EVERY SINGLE feature PLUS added features + ONLINE, website forums and all! But I cant figure out how to make inventory similar to this one, with Quantity. I bought 80% of the inventories in the store but non have stackable items, or are simply way to hard to use. Idk if anyone can help in any way then please do it will be greatly appreciated!!
There are a few videos on YT showing how to create inventory systems in C2. I can't speak to their quality, but maybe you should check some of them out if you haven't already.
Since I found it a bit annoying to create inventories within the event system myself, I created a dedicated plugin to help with this task: Inventory Aid. It definitely has working stacks but beware it's not fully finished (version 0.81) and I do not offer it for free unfortunately (currently still C2 only).