My current working theory for all HP magic is that if you think it'll work (and you aren't an idiot) then believing will help make it work.

Maybe Riddle couldn't make more horcruxes because DD convinced him he couldn't go past 7, or he read something that convinced him, or he convinced himself that 7 is so awesome anything more or less than that wouldn't work.

There's more than a few other threads on how magic in general, and horcruxes in particular, work, so without re-hashing all the arguments that have already gone before I'll just say that there are enough holes in the observed behavior of magic that any discussion about it simply can NOT lead to an answer.

It might lead to plot ideas, or an enjoyable discussion, which are both worthy goals, just don't go expecting an actual answer to these questions.

And in the name of whatever you hold holy, don't ask JKR. She'll cook up some horrid half-assed answer that will simultaneously drive us insane with its stupidity and mercifully kill us with its inevitable random additions that will only make more questions.