Currently, the word, "key", usually means a book that is popular and expensive.
Comic book series of hundreds of issues have keys because runs are hard or expensive to assemble or the story or mythos of the character tends to become repetitive or directionless after decades of writing/drawing. First issues, iconic covers, and introductions of important characters/villains are the most recognizable type of keys.*
Therefore for DD, #1 and #7 are the two traditional keys. Throw in the early Romita Spider-Man issues, #16 and #17, and you will have a collection of four issues with which not many will argue.
If you want more from the Silver Age DD series, you might approach it from a literary/artistic perspective and ask yourself which books are important to the DD mythos or story. You would be personally defining the series by: writer; artist; character introduction and development; and plot direction. For example, you could pick up a book and explain why it is important according to one or more of these criteria.
This particular approach would mean many of the early books will be important, with the number selected becoming less and less as the series becomes more established. Such a collection could easily contain 20-25 of the first 100 books.
*though first issues and covers have become more important as less collectors are interested in reading the interiors due to slabbing or quality of the story.