Hi
You question is not very clear. You can have multiple products also in one sales invoice, so how would you distinguish at the product level. Also, what would be the distinction for determining the region. You may bill to a customer centrally, but delivery can be across to various regions in a country or to a seperate country. You have to first determine these combinations.
That said, if you are clear that sales invoice no has to be different based on for eg, region of the delivering address, you have to do an small enhancement, calling up the user exit in VF01.
USEREXIT_NUMBER_RANGE (Module pool SAPLV60A, program RV60AFZZ)
Thanks & regards
Sanil Bhandari