The Avery Number is used by the label report wizard to set up the
column width and label height for that particular label, and to set it
to 2 or 3 (or more) columns.
The Avery Number is irrelevant once you have created the label.

If you wish to purchase Avery labels, then just create a new label
report using that Avery number when the label wizard asks for it.
Once you already have a label report set up, all you need do is change
the detail width and height as needed.
Then perhaps, modify the label controls if needed.
If more modification is necessary, in the label report design view,
click on File + Page Setup + Columns.
There you can change the number of columns, set the distance between
label columns if needed, as well as change the column width (by
unchecking the "Same as Detail" check box and entering your own column
width value).
Experiment on a copy of the label report.