Currently Being ModeratedMay 14, 2013 9:40 AM (in response to bryan135)
Apple Mail probably has no limit, but in addition to any limits that may be set by your service provider, emails with very long lists of addresses (like hundreds or thousands) may be filtered as potential spam by some of your recipients' mail servers. If you are concerned about the number of recipients you can use a mailing list service like Constant Contact or Campaign Monitor that can help format emails so that they avoid tripping spam filters.
But if you are getting an error message about validity, it is probably not about a numerical limit. It is more likely to mean that one or more of the addresses is not formatted correctly, like a letter or symbol is mistyped in a contact record in Address Book.