Length of an email list in sent mail
I'm not sure if there is a hard limit in Mail for the number of recipients in any of the particular fields. The larger obstacle may be your mail service blocking the message because of the number of recipients: their spam filters tend to flag messages as spam when the number of recipients goes over a certain number.

What you can try - and I have had this work - is to use an Address Group. Put the contacts you are sending to in a Group in your Address Book, then enter then name of the Group in the To: field. It masquerades the email addresses to the recipients and may get it past the spam filters.