I know it's not a bad addiction

It is quite bad. 8 fingers of KitKat give you:

430 calories
22 g fat
(of which) 16 g saturated fat
40 g sugar
Keep that up for a couple of years, and it will have adverse effects on your body, such as elevated triglycerides, LDL ("bad cholesterol"), and blood sugar.

I am going to teach you a technique that is guaranteed to work if you implement it. I used this technique to quit smoking many years ago. I used to smoke 8 (ha!) cigarettes per day. Here is what I did when I implemented the method: Had 8/day for a week. Then 7/day for a week, then 6/day for the week after, and so on, till I was down to 1/day, and then it was very easy to quit. Cutting it down gradually like that acclimated my body to reduced doses over a period of time, while giving me the same pleasure as before. In other words, when I went down from, say 8 to 7, my body/brain did not tell me to have 8 because 7 induce the same effects in my brain. The key is to reduce slowly. Had I stepped down daily, of course it wouldn't have worked. I guess that's how they also do it in drug rehabilitation clinics. Let's apply it to your scenario.

In case of KitKat, it doesn't come in packets of odd numbered fingers, and if you have a packet of 4 fingers, it would be very very hard to eat just 3, for example. If you think you can do that, then simply do what I did, starting at the beginning of next week:

Have 8 fingers/day for a week.
Following week, still buy 8, but have 7/day for a week. This means you will have leftover fingers daily which you would consume the following day.
The week after, eat 6/day. You could get a packet each of 4 fingers and 2 fingers to make it easier.
Keep doing that till you are down to 2 fingers/day.
After that, you can continue having 2/day, as it is not necessary to eliminate it completely. However, if you wish, you can cut it down to 1/day for a week and then quit, and just have a couple of fingers on each day of weekend, for example.

If it would be very hard not to eat 8 fingers when you have them in front of you but you are supposed to have only 7 or 5 that week, then do this:

Have 8/day for 2 weeks.
Following week, buy a pack each of 4 fingers and 2 fingers. Have the 6 fingers/day for a period of 2 weeks.
Then cut down to 4 fingers/day for 2 weeks, or if necessary, 3 weeks.
Then finally down to 2/day, and as mentioned, maintain that or quit.
Whether you step down by 1 finger or 2 fingers at a time, you can adjust the period to what works best for you, either 1 week or 2 weeks. But once you choose, try your hardest to stick with it!

It is also better to space out the fingers throughout the day, instead of having them all at once. If you are eating 8 on a particular day, have 4 during the day and 4 at night. Or even better, have 2 at a time spread throughout your day.

Here is another thing you should do--learn how to savor KitKat properly, at least that's how I think it should be eaten/enjoyed!

Most people I believe bite, chew, and swallow. You end up finishing it quite quickly that way, and it provides fleeting and unrequited pleasure. Once you finish 2 or 4 fingers, you are still not satisfied and feel like having more. This is how I eat it:

Bite a piece off and move it around in my mouth for a while without chewing. Press it against the roof of your mouth with your tongue.
The chocolate melts slowly and coats your tongue and mouth, which is very pleasurable! You actually taste the chocolate, instead of chewing it.
After most of the chocolate has melted and gone, the wafer is left, which is very soft and moist by then.
Then I gently chew the wafer or break it apart with my tongue and savor every bit of it. It tastes and feels very good!
On to the next bite, and so on.
That way, the same amount of KitKat takes around 5 times as long to finish, prolonging your pleasure, and at the same time, letting you savor the tastes and textures of the different components. Chewing converts it into a homogeneous paste.

The 2nd step above, that's how you are also supposed to eat a proper solid chocolate bar. Chocolate, just like coffee, wine, and cologne, has multiple taste and aromatic components, which are a function of the soil, environment, and country the cocoa was grown in. The tastes are in layers, meaning, they are perceived by us not all at once, but different tastes/aromas (called "notes") are released at different times as the chocolate melts. This is hard to experience with the chocolate used in candy bars because they use cheaper chocolate. When you eat a good quality dark chocolate the way I described, it might taste, for example, coffee-like initially as the chocolate starts melting in your mouth. A few moments later it might give way to a citrusy or fruity note, and then perhaps end with a different note. You might have experienced that with cologne if you use it. Initially it has a certain smell, but after half a minute or so you might smell a different note, and then a couple of minutes later, it will smell different again.

Try eating KitKat the way I described and see if it leads to greater enjoyment with fewer number of fingers.

Here are some other suggestions:

Switch to dark chocolate KitKat. At least that way you can get the health benefits of chocolate! You might also find that you are satisfied with fewer fingers as dark chocolate has a more intense and nuanced (has several notes) flavor profile.
Try different flavors of KitKat, such as orange or mint. They taste great, and switching up the flavors regularly might help you quit easier... or make you addicted to multiple flavors all at once LOL!
Switch to eating plain solid dark chocolate, whether now or after you have nearly quit KitKat. Eat the way I described. If you don't like it or are not used to it, start with a 60% cocoa bar (or 55% if 60% is too strong). Move up to 70% cocoa dark chocolate if you like it. If not, stick with 60% and have a few squares a day. It has far less sugar than milk chocolate and than candy bars, and is actually good for your health.
I hope you will take up my suggestions. Since I took the time to help you out, I would greatly appreciate feedback from you after you implement my quitting method (and any of my other suggestions, especially how to eat KitKat/chocolate), regardless of whether it worked or not.

And if you succeed, mail me a few packs of a variety of flavored KitKats! :-)

LOL, I just realized, this is the longest Quora answer I have written so far, and it is about... KitKat!

KitKat... it is a science, and as we all know, science is serious business ;-)

P.S. You can apply the same method to any other addictions you might have :-)