Is there a maximum number of fields that can be sent through an
Outbound Message? I cannot find it documented anywhere.
I see you've tagged your question with workflowoutboundmessage, so assume you're talking about a message to a SOAP/xml endpoint. Workflow, requires a template of some kind, so one couldn't write the code for the message entirely in Apex to send the object record as a class.
That said, I'd expect the limit to be based solely on the limits for custom fields and workflow rules because you could presumably send a complete record update via outboundmessaging. The limit would seem to be based on the limit of max number of custom fields within the constraints on the size of a single message transaction & session.