How many valence electrons in kr?

For the main group elements (Groups 1, 2, 13 - 18), the valence electrons for the atoms of an element are located in the outermost (highest energy) s and p sublevels.

The electron configuration of krypton is##"[Ar]""3d"^"10""4s"^"2""4p"^"6"##. Krypton (Kr) has 8 valence electrons in the 4s and 4p sublevels, which makes it very stable and nonreactive, and this is why it is a noble gas.

Some would write the electron configuration as ##"[Ar]""4s"^"2""3d"^"10""4p"^"6"##

Here is a video which discusses how to draw Lewis structures for atoms showing their number of valence electrons. Video from: Noel Pauller

When atoms react, they share, gain, or lose electrons in order to achieve an octet (8) of valence electrons, except for hydrogen, which reacts to get 2 valence electrons like helium, the first noble gas. This gives the bonded atoms the electron configuration of a noble gas, making them stable.