At the moment, 23.
All are between 1" above knee and just below the kneecap. Don't like them long, nor do I like them really short.
Five are straight, two lined, three not. Of the straight skirts, two are black and are lined. Both are fairly lightweight fabrics, but comfortable. One of those has a 2-3" slit where the left knee is. Doesn't bug me, so I've left it. Of the rest, one's denim and is a typical 5-pocket jeans skirt with the fly placed as it would be for a guy. There's one that is made of a gray knit material, which is kinda clingy unless worn with a slip. The fifth one is navy blue and a little less straight than the others. Also has a couple pockets and a drawstring waist.
I have one that's probably an eight-gore skirt, with the gores near the hem, so it's kinda like a trumpet. This one's salmon pink, so it typically doesn't get worn as an outer skirt, but maybe as an underskirt, underneath one of the other ones. Comes to about mid-knee. Lined, with a somewhat sheer outer layer.
I've three black pleated skirts, ranging from 1" above knee to mid-knee. So, 17-19" long. The 18" long one is usually worn on the outside, since it's almost sheer and needs a layer underneath it to make it less noticeable. The 19" one is heavy, but very full, so it can sometimes be a handful in windy conditions unless I stand a certain way. Typically gets a bit flighty in urban areas where the wind channels between buildings. The 17" one has become more A-line due to multiple washings. Also have a navy blue pleated knee-length that's pretty heavy and unlikely to blow up, pretty much like a kilt. Length is about 21" or so. Was bought at that length in 2006, and hasn't been hemmed up at all. The fifth pleated skirt is a knee-length white/black polkadot skirt. A little small, so will be worn once I've slimmed down a bit. Probably will combine with a black shirt, black knee socks, and athletic shoes to do the two-tone look.
Got two tiered skirts. One is a squaredancing skirt that came with the salmon gored skirt in 2005. Is a tie dye denim blue color. About 20" long. Goes well with a pair of trouser socks and athletic shoes in the summer. Light enough to keep me from sweating, but not likely to do a marilyn monroe on me. The other is a black skirt that's about 22" long. Comes to the tops of my socks, so there's no bare leg showing while standing. Might provide a flash of skin if I'm walking at a decent clip. Does have an embroidered strip of fabric that looks kinda like a lace pattern about 2/3 of the way down. I haven't worn this one recently, since I'd like to get it a bit shorter, so it's not as warm on days that would be appropriate for it.
Brown skirts: have two. Both are fairly full. One's from Old Navy and needs to have the waistband gathered a bit more, so it's less likely to slide down on my body. The other is from Kohl's and is a little on the small side, so won't be worn much until I've shrunk. It fits, but is a little tight. Has a white abstract floral pattern from about 1/2 of the way down, down to the hem, which does have a thin strip of lace. Probably not going to worry about the lace, since it'd be something you'd be hard pressed to see.
Got two more black skirts, one of which is knee-length, a-line, but paneled as well. Not a true pleated skirt, but looks close enough. The other is mostly straight, except for a ruffled hem. Wear that one underneath one of the pleated skirts if I know it's going to be majorly windy.
Patterned skirts: two here. One's a traditional floral print that I've sometimes worn underneath the really lightweight black pleated skirt. Does provide a little bit of color underneath on a sunny day. The other is an abstract pattern in blue, white, and maroon. This one's a little on the small side, so will be worn as I lose weight. Also is a lightweight silk, so should be comfortable in the summer.
One more denim skirt - made by Levi's. Light blue denim, soft fabric, paneled, so a little wider than a-line. Two pockets in the front, none in the rear, belt loops, and the typical fly. Wear that one for bike riding and have worn it when I've been searching for used car parts at the wreckers - no comments from people, plus I don't have to worry about ripping the crotch out while jumping over mud puddles.
Also have a couple navy blue minis that started out as bottoms for women's two-piece swimsuits. One is still complete, so, if I'm doing stuff around the house or the yard, all I need is a shirt, the skirt, socks, and, if outdoors, shoes or boots. The other isn't, so it requires underwear, and will probably be retired anyway, since it's tattered.
Until yesterday, I had four more, but one was a mini that probably was 15" long, so it showed too much thigh, plus was made for someone bigger than I, so it didn't fit right. Also had a very wide waistband, then short boxpleats. Prefer more pleats than waistband. Also purged a black velvet pencil skirt, a blue pinstripe pencil skirt, and a very nice brown skirt that was about ankle length on me and covered with a net-like fabric. If that one had been knee-length, it probably would've seen wear, but, alas, no dice. So, it's gone.
Of course, I still have the three tartan kilts I got in 2002 and 2003. Don't wear them much, since they don't really fit properly anymore. Gained weight.

The rest of my skirted clothing was purchased between June 2005 and probably March 2010.
-J