The possible number of DataSetMessages for one DataSetWriter in a NetworkMessage depends on the KeyFrameCount.
KeyFrameCount = 0 -> number is 0 to N
KeyFrameCount = 1 -> number is always 1
KeyFrameCount > 1 -> number is 0 or 1

The DataSetWriter sends DataSets. A DataSet is described by a DataSetMetaData and this is defined by a PublishedDataSet. A PublisedDataSet also defines the source of information (if visible in OPC UA).

There are two well defined sources (1) a list of OPC UA variables and (2) OPC UA events.
Other sources are application specific and also the configuration of the source is application specific.

For the two OPC UA defined sources, the allowed KeyFrameCount values are clearly defined:
(1) PublishedDataItems requires KeyFrameCount >=1
(2) PublishedEvents requires KeyFrameCount of 0

Therefore a PublishedDataItems PublishedData set can produce maximum one DataSetMessage per PublishingInterval. In UADP it produces either a Data Key Frame DataSetMessage or a Delta Frame DataSetMessage.

A PublishedEvents PublishedData set can produce one to N DataSetMessage per PublishingInterval. In UADP it produces Event DataSetMessages.

For custom sources, it is not defined what kind of KeyFrameCount they allow and also not what kind of UADP DataSetMessage they produce.
It would be good if we would define the allowed KeyFrameCount as property for custom PublishedDataSets.

Would the KeyFrameCount automatically defined the UADP DataSetMessage type?
1 - Data Key Frame DataSetMessage (Yes)
higer than 1 - Data Delta Frame DataSetMessage (Yes)
0 - Event DataSetMessage (not sure - currently event DataSetMessages are limited to Variant field encoding)