An attempt to answer with half a million atproto repositories & two coin flips.

Implementations should limit the number of TreeEntries per Node to a statistically unlikely maximum length.

What actual number should we pick for this maximum?

"Several hundred" seems to be the answer, and with the maths to back it up, too.

That's assuming a fanout of 4. I don't know how Bluesky chose this number, and I haven't tried to work out how suitable it is for my own project.