Leith's suggsetion stores the number in the registry, which is fine. However, it isn't available if you are using another computer.
This is another way that stores the number in a Name, for a full explanation of using Names in Excel read Bob Phillips article here
http://www.xldynamic.com/source/xld.Names.html
To use this you must be using a workbook template, ie a master copy that you save copies of. I usually have a database built in for storing invoice data, etc.
This code will add a name that stores your unique number
Bookmarks