You can theoretically apply for 2 every 65 days, if you can keep up that fast with the $1500 spend required on each card for the 50k. Few people do it that fast, that's just the theoretical limit. (One person in one of the most recent posts in that thread mentions getting this card 10 times over the past 2 years.)

In my case, I tend to overlap, something like get HH card 1, get HH card 2, get HH card 3, cancel card 1, cancel card 2 maybe, get HH card 4, cancel card 3 maybe, etc. In other words, I don't cancel before applying for the next one, but I cancel eventually, so I don't have too big a "useless" stack. (How quickly I cancel may depend on whether there's credit limit on the card I no longer use that could be moved to a new card if that's needed to get a new card approved. Once you cancel, though, any credit limit from the cancelled card doesn't return to your "pool" until 3 to 6 months later, which is why it's better to cancel after you apply for the next one rather than before.

But perhaps not everyone does that.

Again, please go and read at least the wiki in that thread and the last few pages. All of your questions have already been answered there many times over!