Can I send an email with 300 bcc copies from my iclud.com email account

Hello and welcome to Apple Support Communities, Janerik49.
If I understand your post correctly, you'd like to find out if you can Bcc 300 people in one email from your iCloud email account. I can understand wanting to send once and be done. I'm happy to help you get more information about this.
This article shows the limits of an iCloud account:
Mailbox size and message sending limits in iCloud
This section shows the limit on recipients per message:

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 (200 messages)
• The number of recipients you can message each day (1000 recipients)
• The maximum number of recipients per message (100 recipients)
• The size of incoming and outgoing messages (20MB, up to 5GB with Mail Drop turned on)

Thanks for using the Apple Support Communities. Take care.

When you exceed message sending limits, you might see the following alert messages depending on how you view iCloud Mail:

• On your iPhone, iPad, or iPod touch: A copy has been placed in your Outbox. Sending the message failed because you exceeded your sending limit.
• On your Mac: Cannot send message using the server iCloud.
• On iCloud webmail: 550 5.7.1 membername@icloud.com: You have exceeded your daily sending limit. Please try again later.

Thanks for using the Apple Support Communities. Have a great day.