You should only need one csv file. In fact, it is probably not necessary to have the point coordinates anywhere but the stl file if you are using Append Attributes. Table to Points will want to assign point coordinates, but they will just get ignored by Append Attributes.
For the second question, if you are using Append Attributes, then yes the number of points in your csv file must be the same as the number of points in your geometry (and in the same order). If you are using Resample With Dataset, then you do not need to have the same number of points (but you will need point coordinates). That said, Resample With Dataset can get flaky with finding the points (for reasons I won’t get into).