How many times do Ты think a person needs to read Harry Potter before they can completely understand it?

Depends completely on the person. Some people ate able to comprehend things faster then others. Some people have trouble reading. Some understand sifi или romance better then fantasy. Everyone's Чтение style is different.

Once

I think if Ты read it once Ты will understand it one way, if Ты read it twice Ты can pay attention to things Ты did not pay attention to first time Ты read it and maybe see a lot of things differently. Because HP has depth and layers.

However many times the person thinks necessary.

I only read the series once and I'm pretty certain I got everything down. There hasn't been a single thing that anyone has mentioned that I didn't catch. It all depends on how much attention Ты pay.

Just once, it's not that complicated.

once. but it never hurts to read it over and over (and over...) again :D

Just the once.