You'll need to consider your table structure before looking at your needs in terms of layouts. There've been several threads in this forum that deal with an "inventory log" approach that would seem to be a good fit for what you want here. You should be able to use the forum search tool to find them. The basic concept is to treat inventory like a ledger where each change in inventory is logged like a deposit or withdrawel from a bank account and calculations and summary fields then compute the current "balance" to tell you how much of each item you have on hand.
A summary report based on this log can then tell you current levels for each item, grouped by your categories. Other reports and/or charts can be developed that also tell you how your inventory levels change over time which is useful to many businesses.