running number in a cell every time click 'keep' button.

i make first sheet as a receipt. the function is to copy details of payment into any other selected sheet (48 sheet).
i would like to have a running number in sheet1,b12 and will be updated into new number everytime i click
"simpan"(keep) button.

also i need to begin the number as the number on current actual bill book. for instant, i need to begin with 6500

i attached a sample to explain the situation.

thank you.