how many addresses can I include on a single email?

Hi Brian,

Thanks for reaching out to Apple Support Communities. It sounds like you have a question about your iCloud email account. We're happy to provide some information to assist.

You can find helpful information in the article here: Mailbox size and message sending limits in iCloud

Specifically the section that states:

Limits on sending messages 

There are several safeguards in place to make sure that only iCloud members can send messages using iCloud. One of these safeguards includes reasonable limits on:

• The total number of messages you can send each day (1000 messages)
• The number of recipients you can message each day (1000 recipients)
• The maximum number of recipients per message (500 recipients)
• The size of incoming and outgoing messages (20MB, up to 5GB with Mail Drop turned on)

iCloud Mail service is designed primarily for personal use. Sending unsolicited bulk email messages through iCloud email servers is prohibited. To review the iCloud membership agreement and acceptable use policy, please see the iCloud terms and conditions.

Best!