its not the easiest question, by any means - as there is no such thing as a row number in access - its a bit of a tautology in fact, as the row order (and therefore the row number) depends on how you have currently sorted the rows.
if you need an absolute row reference thats a different thing, but you need to include that in your table, and again there can be issues - eg what if you delete an item from the middle of a sequence
so what you normally have is a unique key (ie no duplicates allowed) that will sort the data into a specific order - then you can count those rows, but numbering them is more problematic, as we have seen