The importance of stock checks

I own over 500 ProMarker pens. I have one set at home to work on commissions, do own drawings. In the shop I have a set in marker wallets and a drawer of ‘spare pens’ which art for fun participants are occasionally able to see.
I never have a complete set of all the pens available and to be honest that is never realistic as these sets are used both by me and my art for fun participants and therefore wear out from time to time.
It is the time of year, ie Black Friday where good deals are available and I need to consider which pens I buy / replace. I used spreadsheets a lot in my previous career as a teacher and find them useful as a planning and monitoring tool for my business for finances, stock of art items and in this case the equipment to produce my art.
How do others keep on top of art material stocks?
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