How to generate sequecnce Number?

Question

Hi I am developing windows application..i have 1 combo box containing Ford,honda,,,list items..and i have two label one for displaying date another one is to generate sequence number(from 01-50)....

if i am selecting ford 5 times with todays date..the sequence number should be generate 01,02,03,04,05....

then i m selecting <honda> 5 times With todays date the sequence number should be generate 01,02,03,04,05.....

if i m selecting <ford> 5 times  on tomorrows date the sequence number should be generate 01,02,03,04,05...same for <honda> also.

Can any one tell me a logic.....