well derbyshire is 1, then they follow alphabetically with the number.
but if you edit it it says at the top:
(original club) & derbyshire
so you are in effect changing their nationality to a club.
i don't know what number to edit to change their club, and if for example you change pietersens number from 44 to 1, at the top of his page it will say hampshire & Derbyshire. if you do this the player also doesn't join your club and they can't play for a country so its pointless
Full list of county numbers:
derbyshire -1
durham - 2
essex - 3
glamorgan - 4
gloucestershire - 5
hampshire - 6
kent - 7
lancashire - 8
leicestershire - 9
middlesex - 10
northamptonshire -11
nottinghamshire - 12
somerset - 13
surrey - 14
sussex - 15
warwickshire -16
worchestershire - 17
yorkshire - 18