HP 15.6" AMD Quad-Core 500GB Touch Laptop

Does it support more RAM? What is the maximum amount? How many slots?

According to HP’s own documents:
https://support.hp.com/us-en/product/hp-15-ba000-notebook-pc-series-touch/10862319/model/12344106/manuals

… “Two non-customer-accessible/upgradable memory module slots” and “12288-MB total system memory (8192×1) + (4096×1)”.

(Emphasis added to non-customer-accessible – so you’re on your own here.)