Are Peppol SMP entries by Trading Partner or by Document Type?

1 entry per document type, not per trading partner.

Peppol's SMP registration is keyed to the participant + document type, not to any specific sender/receiver relationship. That's the entire architectural point of the four-corner model: an SMP is the directory entry describing what a participant can receive and where to send it, and the SML is the global index that tells a sender which SMP to ask. The SMP publishes information about where a participant can be reached and what document types they support — it's not a per-relationship record.