Mandatory Cell Input

Hi,

I have an Excel file with Column Headers in it. This Excel file will be fill in with few rows of data by another party.
Several columns are required. But not all are required.

I know how to make a VBA for mandatory cell input but only for specific cells. The idea is that I don't know in advance how many rows the the other party will make. Basically when the user starts filling the first row only then specific cells from that row are required in case that the cells are left blank( by a message box or something similar-without the user to have the possibility of saving or closing the document without filling the cells required)
As I mention I have no information about how many rows the user will fill therefore somehow it has to be dynamic.

If someone can help with some ideas that will be very much appreciated.
Thanks in advance

Thank you