One note, one idea. Not one note per source, per session, or per topic.
The reason is mechanical rather than aesthetic. Links connect notes, so the note is your unit of connection. If a note holds three ideas, anything linking to it links to all three, and the link stops meaning something specific. The graph fills with edges that say “these are vaguely related”, which is the same as saying nothing.
The test is whether you can title the note as a claim. “Reading notes — March” is a container. “Backlinks change what you write” is a claim, and other notes can agree or disagree with it.
Splitting is cheap and merging is expensive, so split early. See [[Evergreen notes]] for why revision matters, and [[Backlinks change what you write]] for what you get back.