To answer the question in the header: too damn many. In the realm of quixotic endeavors, trying to get people to use the word "anniversary" correctly ranks as one of the most daunting. It's up there with "hopefully" in the frequency with which it's wrongly deployed. (Even the AP Stylebook editors threw up their hands in futility over that one in 2012.)

As someone who works in the media, I see phrases like "10-year anniversary" (to cite just one example) nearly every hour in press releases. One will also find such misuses in Facebook event pages, Twitter, newspapers (even this one), blogs, and anywhere else people keep track of annual landmarks. This is just as painful to anyone who values linguistic accuracy as hearing somebody say "ATM machine." It's death by a hundred-thousand cuts.

Here's the hidden-in-plain-sight secret about "anniversary": the word "year" is baked right into it. You don't need to add "year" to "anniversary." Breathe a sigh of relief. Check any dictionary for the word's root, and you'll find the first part comes from the Latin annus (stop snickering back there). So, instead of writing/saying "10-year anniversary," just write/say "10th anniversary." If you add up all the seconds you'd've saved over the years by not writing/saying that extra word, you'd have enough time to read Infinite Jest... and still have a social life.