When using Spreadsheet uploads for "Add Bills to Existing Meters" you can add bills for multiple meters and multiple properties. Portfolio Manager asks you how many bills you want to add for each type of meter. However, you may not have the same amount of bills for each meter of the same type. For example, say you have some electric meters for which you have 12 months of bills, and some electric meters for which you only have 10 months of bills.
When you create your custom spreadsheet, say you have "12" months of data. When you are filling in the spreadsheet, if you only have 10 bills, you can delete the extra 2 rows or leave them blank.