What is the maximum number of contacts in a group that will send in an iCloud email? I'm sending from MacBookPro.

100

See: http://support.apple.com/kb/HT4863

Limits on Sending Messages

iCloud has several safeguards to ensure that only iCloud members can send messages using the iCloud outgoing mail server. Among these safeguards are reasonable limitations on:

• The total number of messages you can send each day (200 messages)
• The number of recipients to whom you can send each day (1000 recipients)
• The maximum number of recipients per message (100 recipients)
• The size of incoming and outgoing messages (20 MB)

You can send 10 emails to 100 people each in a day, for example, or 198 messages with, on average, 5 recipients (and five ninety-ninths).