Let me just say that ornaments have a way of creeping up on a person, like pounds or sets of china. If you hit the stores every year right after Halloween, when the Christmas stuff starts pouring in, and then again at dawn on December 26, to see what's on sale—even if you buy only a few things on each foray, if you keep at it for, oh, say, 50 or so years, you'll wind up with the kind of wildly overdecorated tree that makes guests at your annual Christmas party go, "Wow! How many ornaments do you have?" or, more pointedly, "Omigod! How long have you been collecting?"
In answer to the first question, I don't know; I keep losing count. In answer to the second, forever—but I wouldn't call it collecting; I'd call it shopping.