Excerpt from Outlook 2007 Help:
The maximum number of names that you can include in a distribution list
depends on the size of the contact files (for each contact). In general,
for users of Microsoft Office Outlook 2007 Service Pack 1 or earlier, a
distribution list can contain between 50-70 names and e-mail addresses.
2007 Microsoft Office Suite Service Pack 2 (SP2) removes any limitation
to distribution list size. If your computer has Office 2007 SP2
installed, you will be able to create and use distribution lists with as
many names as you want.