Can I send 400 emails in a group distribution list?

See the Mail User Guide on how to send email to a group (without their individual names appearing). You should also consult with the Contacts User Guide for how to create a group for that mailing.

I suggest you try a small (5 people) group mailing to see that it works. A mailing of 400 people may have the undesirable effect of showing up on your Mail ISP's radar as a potential SPAMMER, if they do not already have a limitation on group mailing size. If this is a work from home scenario, you might consider using the corporate Mail server for corporate business.

Otherwise, Apple does not publish the maximum size of a group mailing, and it may or may not work.