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
No,the currency supply shouldn't be equal to the GDP.There was a simple formula about that topic,but I have to search through my Macro-economy and finance books.
As far as I remember,the classic equation was M.V=P.Y or M=1/V.(P.Y)
Irving Fisher was the author of the classic quantity theory of money.You will have to read more about him.