It's under Page Set-up

What version Publisher?
In 2002-03, select business cards in the page setup, you needn't a template number if
the card is the standard 2 x 3.5.
Click change copies per sheet, set the top margin to .75, side margin .5 and the gaps
to zero. This will result having 10 cards.
In Publisher 2000 and below, you change the margins and gaps when you finish creating
your card and go to print. Click Page Options, click Custom Options.

Here's I do it with Publisher 2000.

File
Page Setup
   Special Size
   Business Card from the drop down menu
OK

Create card.

Assuming you are using standard business card stock of 2 wide and 5 high;

File
Print
   Page options button
       Print multiple copies per sheet
       Custom options button
           Margins = .5"
           Gap = 0"
           Side Margins = .75"
           Vertical gap = 0"

OK
OK
OK

And it's printing a sheet of ten cards.
Save the file and all the settings will be saved for the next batch of the
same cards.

Best done in MS Word using the Avery Wizard..