Question

what is the formula to know that how much double bonds are there in a compound?​

Answer 1

Suppose you've been given an unsaturated hydrocarbon.
Then, Just make the alkane corresponding to the same number of carbons that of in the given hydrocarbon.
Then, take the difference of the number of hydrogens in the corresponding alkane and the given compound and divide this number of Hydrogens by 2 so you'll get number of double bond or what you call the double booked equivalent.

Or, you could use a direct formula in any organic compound
Double Bond Equivalent = C-H/2+N/2+1

However for inorganic compounds and other compounds there's no way other than memorising their structures.

I hope you're Santusht with the answer ! ^_^