Lets assume you have nearly all the glyph recipees, and want to make 1 of every glyph - lets call it 300 glyphs. Cheap enough right? Lets say that you can make a glyph for 10g (even if you can't yet, you will be able to sooner or later). That is still 3000g tied up; and 300 bag spots.
Ok, instead you decide you want to hold a stack (20) of every glyph : That costs 20 * 3000g, or 60,000g, but still 300 bag spots.
As well, you keep a ready supply of inks to make another 500 glyphs on demand; 5000g plus 75 bag spots.
You also buy cheap herbs that you see of every type - a few thousand of the best herb from each ink type would be nice. More gold, more bank space.
One limitation is obviously space. Many dedicated glyph makers have a bank alt or 3, kitted out with bags of endless pockets; and a banking guild; simply to cover the bag space requirements. You could do something as cheap as have an alt with 50 incoming mail with 12 stacks per mail to store glyphs, inks or herbs.
The next limitation is gold. Some players have a lazy 100,000g to pick up a glyph business, but by no means all.
What you want is to have enough stock to have at least 1 of your high value glyphs on the AH at all times. Personally, my current desired stock levels are 2 of each sellable glyph on the AH, with 12 glyphs in my bags, but this is likely to change again soon.
The desired stock levels of a particular glyph/herb/ink are just enough so you don't run out of it.
- Are you happy to mill, and scribe every time you need a glyph? Then you don't need many glyphs, just enough to keep your levels on the auction house.
- Will you mill/write glyphs once a week? Then you still want enough of your best selling glyphs to keep the AH supplied, but this might be as many as 5-7 times the number you post (eg if you post 2 of each glyph at a time, once a day; then you may want a stockpile of 10-14).
- Are your ink or herb prices and supply consistent? No need to stockpile many of these
- Are there wild price changes, sometimes running out entirely? Then you may wish to have a stock of 1-2 weeks.
It is also not ideal to have 6 months supply; depreciation (falling prices) hurts when you have too much stock.