Question

Chemical bond formed by sharing of how many electrons

Answer 1

As many as it is required to fulfill it's octavalency. (8 electron stability) 

there are some exception though. 

Answer 2

Chemical bonds formed by sharing of electrons is generally called as the covalent bonds....They are shared in such a way that the both elememts should satisfy their valency to gain stability among them and form a best molecule as possible...so it is not fixed ......on number of valence it always varies....