I've got some better numbers, Macruran. Accuracy not 100% but pretty good.
PlaymoDB has 4328 sets in the DB. That's only sets with 4-digit numbers - I don't use the old Schaper three-digit numbers, or the ten-digit ones assigned to new "gift" sets like the PCC produces. 238 of the four-digit numbers are used twice or more. So that's 4090, out of 8999 possible.
In the DB...
1000's: 14
2000's: 1
3000's: 964
4000's: 977
5000's: 835
6000's: 497
7000's: 784
8000's: 0
9000's: 21
Before you check my work, that only adds to 4079, but I don't know where the other 11 went! Some kind of weirdness with the doubled numbers I expect. Anyway...
The 7000's were used for DS sets for many years, and 6000's for 1-2-3 sets; that's not true anymore, so new sets of all kinds are almost always 5000's and 6000's. The Specials still get 4000's till they're all gone, I suppose. The 5000's won't be long getting used up, either, 2-3 year I'd wager.
There have been over 100 sets a year since 1996 with only a couple of exceptions slightly lower; over 175 sets a year since 2005. 2012 and 2014 were high points at 241 and 219 sets, respectively... and of course 2015, already at 182, is not done yet.