You're not really giving us a whole lot to go on here.

Does your Excel sheet have the membership card number in it? If so, add the
appropriate field to the mailmerge main document and format it the way you
want.

if not, maybe you are asking how Word Mailmerge can assign a sequence number
to each card? If so, it's probably not a good idea - surely you want to know
which card number has been assigned to each member? In which case you need
to assign a number to each member before you even do the merge? There are
ways to do it, of course, but mailmerge won't remember which number you last
used, etc etc. You have to do that...

Peter Jamieson

See http://www.gmayor.com/Numbered_labels.htm