Spare part readiness:

Producing spare parts is not as easy as it sounds. The quantity of parts required and if it is currently produced for any existing models, all count towards the production of required parts.
We can prepare for the spare parts depending on the designed life of the part, life of the component and current reliability of the part.
Example: Consider the example of a regular "Pen", with a refill in it. The life of the Pen would be say for 2 years and the refill would last for 2 months for all positive conditions, which would mean each pen would require 12 refills. Here we consider the fact that the pen lasts its life time. However, some get damaged much before that and this would not hold good. Here we would have to look at the reliability and current sale of replacement ink refills to decide on the future requirement of spare parts.