Dear Jamie Torgler
Good day! Thank you for posting to Microsoft Community. We are happy to help you!
As far as I know, there's only Address books limit in Office 365. We don't have any official articles about contacts limit in Office 365. So it's not necessary to worry about this. I've seen much more contacts in other cases. There is no definite limit to the number of contacts that you can add to a distribution list. The exact limit is based on the total file size of the contacts. If you use only the very basic information for contacts, your distribution list limit may be as many contacts as possible. The typical number of contacts that you should add to a distribution list is between 50 contacts and 70 contacts. However, if the contacts contain lots of information, the distribution list limit should be as small as 25 contacts. In addition, there is a recipient limits when sending emails, you can only send an email with up to 500 recipients. In the journal of message delivery, the contact group (formerly distribution list) will expand its members in field. For more information, please check: Receiving and sending limits.
Thanks in advance for your understanding! Your patience and cooperation will be highly appreciated. Hope you all the best!
Sincerely,
Kerry Chen | Microsoft Community Moderator