Create a formula field in Lead object as below.

Formula Code:

IF( ISBLANK(FirstName ) , 0, 20) +
IF( ISBLANK( LastName ) , 0, 20) +
IF( ISBLANK( Email ) , 0, 20) +
IF( ISBLANK( Phone ) , 0, 20) +
IF( ISBLANK( MobilePhone ) , 0, 20)

Cheers!!!