Of course the simple answer is to create a custom N:N relationship between contact and appointment and store your attendees there.
But I am interested in this error. This is a proprietary attribute type and MS doesn't say much about its' limitations. They also do not let us use this attribute type in customizations, and this may be one of the reasons. But, there doesn't
seem to be any logical reason to limit the number of entries unless this field is based on a completely different SQL schema. The other possibility is the HTML rendering engine in CRM is not equipped to deal with more than 13 entries.
The 404 implies that the web server is looking for a page or resource that does not exist. When and where, exactly, are you seeing this error?