Hypoltan,
As pointed out above by
DSzymborski, there's no way to predict how well your particular i5-9600K will overclock; all we can say is what's typical. Depending on the quality of the sample you receive, your results can vary by +/- 200MHz.
Overclocking is always limited by two factors; voltage and temperature. Like diamonds, no two processors are identical; each has unique imperfections that determine voltage tolerance, thermal behavior and overclocking potential, which is often referred to as the "silicon lottery".
If you don't want to roll the dice, and you're instead willing to purchase a new CPU that's been professionally delidded, tested and binned for a specific overclock, then go to a reputable company, aptly named
Slilicon Lottery -
https://siliconlottery.com/collections/all/products/delid?variant=12794952450134
They do a great job and provide a warranty, along with an overclocking report. The report shows the conditions and BIOS settings you'll need to configure in order to achieve the same results, so there's no guess work or trial-and-error as is typical with overclocking.
When overclocking, as Core speed (MHz) is increased, Core voltage (Vcore) must also be increased to maintain stability. This increases Power consumption (Watts) which increases Core temperatures.
Here's the maximum recommended Core voltage per Microarchitecture from 14 to 65 nanometers since 2006:
Here's the nominal operating range for Core temperature:
Core temperatures above 85°C are not recommended.
Core temperatures below 80°C are ideal.
With high-end cooling you might reach the Vcore limit before 85°C. With low-end cooling you’ll reach 85°C before the Vcore limit. Regardless, whichever limit you reach first is where you should stop.
Remember to keep overclocking in perspective. For example, the difference between 4.5 GHz and 4.6 Ghz is less than 2.3%, which has no noticeable impact on overall system performance. It simply isn’t worth pushing your processor beyond recommended Core voltage and Core temperature limits just to squeeze out another 100 MHz.
Beginners guide to overclocking your CPU -
http://www.tomshardware.com/faq/id-3761568/beginners-guide-overclocking-cpu-explicit-testing-guidelines.html
Intel Temperature Guide - https://forums.tomshardware.com/threads/intel-temperature-guide.1488337/
CT 😎