Question a 6600k for be stable at 4.4ghz what vcore must have?

Update your post to include full system hardware specs and OS information.

What does the CPU documention indicate?

For the most part you will probably need to make incremental changes upwards - when stability fails you will know the answer.

And remember that there are always trade-offs. Overclocking may gain little in performance at the cost of reduced component "life".

Start here:

https://www.tomshardware.com/how-to/how-to-overclock-a-cpu
 
There's no absolute value in which it's stable. It depends on the end result when the processor was manufactured

The only way to test if the voltage setting is stable is to start with something that's below the maximum voltage allowed (the interwebs tells me ~1.4-1.425V is the safe maximum but do your own research), see if the computer boots, then run a CPU stress test like Prime95. If it doesn't crash, it's stable. If it crashes, you need more voltage.