I'm like Aida: my inventory is physical/mental rather than spreadsheet/photo based (and I have a large wardrobe). I have two accordian-door, reach-in style closets: one has a rod, to which I added an extra hanging rod across about 1/3 so I could put skirts beneath shirts (It was cheap at Container Store & you just hook it over your existing rod & can adjust height/length), while the other is shelves. Each is about 4 feet across. In the hanging one, I keep my dresses/tunics, then woven blouses & a couple thicker knit ones sorted first by sleeve length (sleeveless/short/three-quarter/long) then by shade (starts with white and goes through navy blue). On the second rod underneath the blouses hang my skirts (sorted by shade, as they're all the same style) and my blazers/indoor jackets. They're a shelf above the rods that holds my bags (I only have 4) and jewelry.
The shelf closet holds more than just my clothes, but I mainly separate shelves into clothing/non-clothing. One shelf holds folded and stacked trousers, shorts, and knitwear so I can see everything at once. The arrangement varies between winter & summer, but right now I have wool trousers/skinnies (which are too hot for me to wear during our summer) in one of the corners, then warm weather trousers, then shorts. The other two-thirds is devoted to knitwear: warm weather cardigans, warm weather pullovers, and then in the other corner are all my wool sweaters. Basically, I put whatever's most weather appropriate in the middle of the shelf as that's most accessible. My layering cam is & regular tees fold up into a little shoebox (stored vertically, like a filing system) and live on a shelf that also has boxes for socks, tights, undies, and pjs. Because it makes me happy, all of these categories are also sorted by shade, lightest to darkest. All of my shoes fit on the floor of this closet. My coats live in a hall closet so are separate.
Because everything is visible at once, I don't really forget my inventory. If I thrift too much and find the stacks are getting unwieldily or I've having to add lots of hangers to my rods, I re-assess and will pull the least-loved items out and/or cut way back on shopping for awhile. I don't mind having a big wardrobe (it's over 200 items I'm sure if we're including shoes and outerwear), but it all needs to fit neatly and easily into its assigned space. I'm pretty much aware of the (very few) 'holes' in my closet and just keep mental track of them, but I also think keeping a shopping list like Aida does would be very helpful. If I need to know exact numbers (usually for a YLF discussion, lol), I just hop up & count!
Since you want to do an inventory to sort out/take control of your closet, if I were you I'd count by function. Like, do you wear your sleeveless summer tops one way but your long sleeved ones a different way? If so, it might be helpful to count them separately. If not, just lump them together. Ditto for work/play, dressy/casual, etc. If you live in a four-season area, divide by that, but if it's the same weather year round, only divide if you wear different colours in different months. Basically, you want your inventory to reflect how you use your clothing, so that you can see in what areas you have more than you need, what areas you have less, and what areas are just right. If you only have one formal skirt, for instance, and need to dress formally occasionally, you don't want to get rid of it just to make your closet smaller! Whereas if you have thirty party dresses and only go to three parties a year, you could probably afford to weed out a few. Anyway, if you do a spreadsheet, you can note all of the different categories that matter to you as you go and then sort them by different group to see numbers that way. If you only have a couple relevant categories though, pen & paper might be just as easy.