Nothing is stored in the Outlook Address Book. All data is stored in your
Contacts Folder, so those limits are set by your Outlook version and data
store, neither of which you mentioned.

Outlook 97/2002 PST size limit is 2 GB. In terms of Item number limits:
16,000 items per folder when large table support is turned off (default for
Outlook 97/2000)
65,000 items per folder when large table support is turned on (default for
Outlook 2002)

16,000 folders per folder when large table support is turned off (default
for Outlook 97/2000)
65,000 folders per folder when large table support is turned on (default for
Outlook 2002)

Outlook 2003 (Unicode) PST size limit is unknown, but practical limitations
are said to be around 20GB. In terms of Item number limits that is also
unknown, but rumor has it at 130 million.  My guess is that you will hit
performance limitations before finding the upper limit on items.