The combining capacity of an atom is known as valency.
If the atomic no of an element is 11.
Then it's electronic configuration will be 2,8,1
Hence it will loose 1 atom complete it's octet.
Therefore it's valency is 1.
Take an other example, if an element has a atomic no 6, then it's electronic Configuration will be 2, 4
So it needed 4 more atom to complete it's octet, therefore it's valency is 4.