Textedit opens every Textedit file
By default, TextEdit (and other Apple apps) will re-open every document if you quit it without closing them first. My suspicion is that you've just quit the app without closing the documents every time, and this has built up.
You can close all the open documents by holding the Alt/Option key and clicking the red button in the top left corner of any one window. Or just use Option Command W to close every window.