datatype how many bytes does each char map to

The answer depend on the DBMS you are working with...

in general

char's use 1 byte per defined length
nchar's use 2 bytes per defined length character
both the above are "Fixed" length

Varchar/NVarchar  have a maximum length defined (and use either 1 or 2 bytes per character)
    there is also a length overhead for variable length columns which is normally 2 bytes for lengths under 4096
    and then will be 4 bytes above...
     (Only the actual used length of the column will be stored)

another modification / factor is whether the column is nullable or not
if null then typically a bit flag in the row will be used to indicate wether the column is null or not (so 1 byte per 8 nullable columns)... I think some DBMS use a similar idea for columns with default values...

so it really does depend on the DBMS (& OS and Version ) your using...