Does static methods and class are good for scalability ? I think so static class/method improves scalability of application and instance methods doesn't scales much. So is it good programming practice to write static method where ever it is possible ?
It depends on WHY the method is static. If it's static because it truly does not need context, then it will probably scale very well compared to something of similar complexity that is not static because it requires context.
However, if it is static merely because you cannot retain the needed context and must still pass it in, or because of some artificial goal of having more static methods, then I would suspect that it will actually scale LESS than the comparable method as non-static.
In fact I think that ASP Classic proved this point.