What is the correct format for numbering tracks?

I'm about to work on a lot of songs and update many of the tags. I see a lot of different formats for the track numbers and was wondering what is the correct / best format to use. It seems to me it should be 1 of the following 2 formats:

nn/nn (example 03/12 which would say it is track 3 of a total of 12 tracks on the CD / Album)

or

nn (03 - just the track number)

Are there pro's and con's of using one over the other. For example, does the track number impact sorting and if so will one work better over the other?

There no correct version but xx/xx is very common.
You can use these variations as you want.
But you can use a separate tag to store the number of the total tracks of an album, if you prefer:
TOTALTRACKS

The number format for the tag-field TRACK depends on the media file type (mp3, wma, mp4, ...)
... this means ...

  • the tag-field TRACK can be of datatype numerical (1, 2, 3, ...)
  • the tag-field TRACK can be of datatype string ...
    e. g. with leading zero (01, 02, 03, ... or ... 001, 002, 003, ...).

For displaying the number value from the tag-field TRACK in a readable format, ...
you may code such a formatstring like this one ...

$num(%TRACK%,2)

... and together with the number value from the tag-field TOTALTRACKS, ....
you may code such a formatstring like this one ...

$num(%TRACK%,2)'/'$num(%TOTALTRACKS%,2)

Keep in mind, that the technical datatype of a stored value can be different from the human readable presentation of the stored value.

DD.20150424.1619.CEST

for tag-field "tracks" i prefer the numerical datatype (1,2,3....)
for the filename of any mp3-file i prefer leading zeros "01 - True Reality.mp3"
it's for better reading in the explorer window.

and i reduce my tag-fields as much as possible.

i only use ARTIST;ALBUM;GENRE;YEAR;TRACK;TITLE
because i have different players (car; CD; usbstick-player etc..) for my mp3-files.
and these standard tag-fields are surely readable by the most
cheaper soft/hardware-mp3-player outside of the windows universum.

by using TAG - FILENAME
i generate the following filestructure to archive my mp3-files

"MUSIC\MP3\Al Stewart\Year of The Cat - 1979 -POP\01 - Lord Grenville.mp3" and so on...

there are only 2 exceptions here. for single mp3-files and sampler-cds like soundtracks etc...
the filestructure is a bit different.

for Singles
"MUSIC\MP3\Singles\01 - Age of Technology (Original Mix) - Croatia Squad - 2013 - House.mp3"

for Samplers
"MUSIC\MP3\Sampler\2001 A Space Odyssey - 1968 - Soundtrack\01 - Also Sprach Zarathustra (Richard Strauss) - Karl Böhm (The Berlin Philharmonic Orchestra).mp3"

just my 2 cents