Question: 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 ?

Answer: 
No static methods don't intrinsically scale better. Infact the programming style (imperative or object oriented) does not really make any difference to scaling whatsoever. There are two major aspects of scaling and what to do to improve scale depends on which we mean:

1 Scaling by number of requests a second handled

This type of scaling is normally about adding more computers to a cluster to improve overall throughput of the system. Increasing scaling is often about initially reducing the amount of shared resources used through the use of caches and then later making the data access split into shards.

2 Data Scaling

This is when the system gets more and more data over time and operations that access the data (search, filtering etc) get slower as the algorithms are more complex than O(1). In this case the normal strategy is to increase the number of read and write points and use parallel algorithms such as Map/Reduce.

But neither of these aspects has anything to do with whether you use static methods or not, just whether multiple requests work on large sets of data or single sources of data.
Voted down. The OP clearly doesn't quite understand OO. An instance method doesn't take up any extra space when an instance object is created. Static methods aren't going to save you anything unless you're also avoiding creating any instances, in which case you're going so far afield from what an OO language was built for that it's sort of pointless discussion.