Window glass can be damaged by many factors. How it is damaged determines whether you can simply replace the glass pane or if the whole window assembly needs to be replaced.
Impact cracks are a result of an impact from outside of the window, such as a wayward baseball or hail strike. Impact cracks typically only affect one window or even a single pane of glass if the window is a multi-pane design. For this reason, you likely won't need to replace every window on the home. The only exception is if hail or stormy weather led to the cracking since this may affect multiple windows.
You can typically repair impact cracks by only replacing the pane of glass that is affected; there isn't a need to replace the whole window assembly. Keep in mind that replacing a single pane instead of the entire window will affect the insulation level in insulated windows.
Stress cracks are most often caused by the settling of the home. They may start off slowly, usually at one corner, and work their way across the glass. Windows in doors are also prone to stress cracking, typically due to the repeated slamming of the door with a bit too much force. In rarer instances, stress cracks may occur due to outside pressure changes, particularly in well-sealed homes.
You must determine the cause of the cracking. If the house has shifted, you may need new frames to account for any warping to the old frame. Glass replacement alone won't solve the problem. You can replace a broken pane in a door, but either the behavior of slamming needs to change or you need to replace the old pane with a thicker, more break-resistant glass.
Weathering doesn't usually crack window panes but instead causes changes in the glass. There may be excessive etching from blown sand, for example, or the coatings on the glass may begin to wear away and create a wavy effect when you look through the windows. The older the windows or the more exposed to the wind they are, the more likely it is that they will become damaged.
It is possible to just replace the damaged glass panes without doing a full window replacement. If you have multiple windows that are damaged and they happen to be insulated, though, a full replacement may make more sense. You will also need to opt for a full window replacement if the frames are also suffering from weathering damage.
Contact a window glass replacement service if your home's windows are showing signs of damage.