According to Gary Garrels in The late paintings, note 24 on page 36, "The Willem de Kooning Conservatorship office has assembled an archive and record of De Kooning's work that traces as fully as possible his production year by year". For the period 1978-1990 the numbers are given in The late paintings. Garrels mentions that between 1978-1980 De Kooning painted little, less than a dozen per year (page 25). In the same book Robert Storr presents the numbers for the years 1980-1990 (pages 42 and 51-52). From these numbers the following table can be derived:

Year	Produced	Reproduced
1978	ca. 10	7
1979	ca. 10	10
1980	ca. 10	8
1981	15	14
1982	28	26
1983	54	39
1984	51	41
1985	63	25
1986	43	24
1987	26	29
1988-90	53	24

The reproduced column contains the amounts of reproduced paintings in the books and catalogues mentioned in the codes list. This list doesn't include all publications, but the major part that has been reguraly available. All publications that are available in the library collection of the Stedelijk Museum in Amsterdam, Holland, are included, as well as the university collections in Holland.
It leads to 247 paintings being reproduced out of 363 produced, which is 68 % (some paintings of the seventies are only roughly dated though, so it remains a bit cloudy). When you look at the coverage of numbered series in other years, like the Untitled paintings from 1975-1977 and the Woman series, it can be concluded that in those years the coverage of De Kooning's oeuvre is possibly even better. There are however some instances of evident gaps in the numbered paintings. These concern the East Hampton series of 1968 (five out of at least twenty-two) and the Untitled series of 1983, where there's little coverage between the numbers XXIV and XLI. The publication of works from 1985 is poor (40 paintings aren't reproduced yet) and of course there are the two years 1989-1990, that because of De Kooning's Alzheimer disease in that period, haven't been published except for one painting in progress.
I'm presuming here that all the titles are correct. Errors and inconsistencies frequently happen in catalogues, which cause two different paintings being listed as Woman in a landscape III in 1968 in KC; two different paintings are listed as no. IV in AR and CP, so I followed the number that gets confirmed in other catalogues. Apart from that, De Kooning himself sometimes retitled his paintings and didn't keep a good record of the dates of his work. Especially many of the drawings can only be roughly dated.
The total number of paintings listed in this site is 933. Extrapolation of the percentage from above would lead to a total output of over 1300 works, but that's just guessing. For the drawings the listed amount is 753. I have no clue about the magnitude of De Kooning's drawn oeuvre. A title as Woman XX from 1958 suggests, combined with the free sketch like character of much of his later drawings, that he could draw rapidly and prolifically. And also that far from all has been reproduced. His sculpture is completely published and consists of 24 original pieces.