If I say that I can work on just one piece and it takes me at least 4 - 6 weeks to "own" it, than that would make 12 pieces a year and maybe 120 pieces in 10 years. But I realize that isn't the way it works. And I wonder why?
It depends on the length and difficulty of the piece. The Hammerklavier is ten times as long and ten times as difficult (=100 x) as a Clementi sonatina, which in turn is ten times as long as the easy Minuet from the AMB book (assuming my math is correct, which it always is
And if you don't keep returning regularly to a piece you own, you'll eventually disown it (or it'll disown you).
Is it because at one point you can sightread most of the stuff that it suddenly goes faster? Is it because you play several hours and work on different pieces during that time? How many pieces would you say you "own"? And ideas to the question welcome!
How much your reading skills improve would depend on how often you use it, which in turn depends on how many different pieces you read (as opposed to learn to play to own) on a regular basis. Good sight-readers (and readers) read through lots and lots of pieces for fun, often with no intention to learn them properly. I probably read at least 200 pieces for every piece I actually learn to play properly, when I was a student. I'd borrow, say, a volume of Schubert sonatas from my high school music library, and read through the whole lot before returning it a couple of weeks later (having photocopied a few of the sonatas or sonata movements that I liked, in order to learn for myself at a future date).
Play every day if you can - even if only for five minutes before bedtime. (You can sight-read something easy in five minutes, or play once through a Clementi sonatina). It's much better to practice half an hour a day, seven days a week than four hours on a Sunday and nothing for the rest of the week.
As for how many pieces I "own" at present, I'd say about 35, give or take 5, lasting anything from one minute to 25 minutes.