Some questions (maybe previously done, sorry)

[Momiji]

No: HPFGUIDX 140759

Hello to everyone!
My name is Laura and I'm a lurker, my questions can be stupid and my
English is bad, but...
Why Dumbledore was so sure about the number of Horcruxes and their
"form" ("important objects")?
Why seven? Only because it's a "magic number"?
Why Voldemort did Horcruxes with that objects? (Too "simple" to find
them.)
Those questions are not mine, but from my friends that did not like
the book, I'd like to explain to them that those aren't "plot holes"
(and also to tranquilize myself about it. ^^;;; )

I'm sorry for my bad English. 
Thanks a lot!

Laura

-- 
"I... I've got a job to do.' 
'Well, then, you must get on and do it, my dear boy,' said Dumbledore
softly."
from Harry Potter and the Half Blood Prince