Will my Battery get harmed if not used for long time?
These are what you need to keep in mind:


The battery works using chemical reactions, these reactions cannot be stopped but they can be slowed.
A laptop with the battery left in will continue using that battery
very slowly.
A battery left completely dead inside a laptop that is still trying
to draw power will kill the battery completely.


If you are storing the MacBook for a few months, take better care where you store it. Place it in a case, or completely dry bag to prevent dust buildup. Even if you keep it in the box it has come with, it would be fine. 

Taking the above care, you don't have to worry about the battery.

Apple's Maximizing Battery Life and Lifespan page offers the following advice:


  If you want to store your device long term, two key factors will affect the overall health of your battery: the environmental temperature and the percentage of charge on the battery when it’s powered down for storage. Therefore, we recommend the following:
  
  
  Do not fully charge or fully discharge your device’s battery — charge
  it to around 50%. If you store a device when its battery is fully
  discharged, the battery could fall into a deep discharge state, which
  renders it incapable of holding a charge. Conversely, if you store it
  fully charged for an extended period of time, the battery may lose
  some capacity, leading to shorter battery life.
  Power down the device to avoid additional battery use.
  Place your device in a cool, moisture-free environment that’s less
  than 90° F (32° C).
  If you plan to store your device for longer than six months, charge
  it to 50% every six months.
  
  
  Depending on how long you store your device, it may be in a low-battery state when you remove it from long-term storage. After it’s removed from storage, it may require 20 minutes of charging with the original adapter before you can use it.