Knuth has kept a very detailed log of all the bugs he has corrected and changes he has made in the program since 1982; as of 2008, the list contains 427 entries, not including the version modification that should be done after his death as the final change in TeX.
The file is called "tex82.bug": http://mirrors.rit.edu/CTAN/systems/knuth/dist/errata/tex82....
Wikipedia is (slightly) out of date since there's 428 currently in the above file, but note that not all of these are actual (functionality-breaking) bugs, just changes; for example, #2 is just a renaming of variables and #425 is an optimisation.
That would be 428 total changes, in a span of a little over 32 years, with the majority of them extremely early in TeX's history - #214 was in 1983, #321 in 1985, #400 in 1991, #420 (a "missing goto") in 2007.