how does the currency supply of a country is decided, I searched got that it should be equal to GDP is this true or it's an ideal condition
The scope of your question is much bigger and can't be suitably replied here. Rather, I would request you to watch the below video from Youtube.
https://www.youtube.com/watch?v=FVXoijm6eZ8This video will at least give you an idea even though money printing and circulation requires a combination of many factors. Hope this helps at least to understand the basiscs!