How many times do 당신 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 또는 romance better then fantasy. Everyone's 읽기 style is different.

Once

I think if 당신 read it once 당신 will understand it one way, if 당신 read it twice 당신 can pay attention to things 당신 did not pay attention to first time 당신 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 당신 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.