How many electrons are transferred in the ionic bond between sodium and chlorine in nacl?

From the periodic table, we can find that:

Sodium has 11 electrons with a configuration of (2,8,1)

Chlorine has 17 electrons with a configuration of (2,8,7)

Based on this, we can see that sodium needs to lose one electron to become stable with valence shell having 8 electrons.

On the other hand, chlorine needs to gain one electron to become stable with valence shell having 8 electrons.

In sodium chloride, the sodium donates this one electron to chlorine forming an ionic bond so they both become stable.