need to pass numeric(20, 8) as a output

This is a formatting issue. You are going get all those zeros ONLY in string representation. The value of numeric representation doesn't change on the presence or absence of those zeros and as such are ignored by language / compiler.

 

Please refer to the The fixed-point ("F") format specifier section in the following link:

 

https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-numeric-format-strings