Those are fairly new events. So I don't know if there is a normal yet.
Diamond Tournaments

I guess my real question is whether there have been many cancellations so far. Also what it the minimum number of players needed to go forward with a tournament? It seems to me that if you had four you could do a three round robin (a "quad"). True if someone withdrew that would pretty much sink the tournament, but if it was a blitz/bullet tournament the players would not be risking much as far as "wasting their time."

I really don't know the minimum to play. The minimum for the rounds advertised is 2x the rounds; need at least 14 players for 7 rounds. The system will do fewer rounds with less players.
My guess is the the minimum is something like half the number needed for all rounds, rounded up. So, a 5 round tourney normally needs 10 players but it will run 3 rounds with 6. Less than that and it won't happen. But that is just a guess.

Tried a fourth one. I was the only entrant. Cancelled. In four attempts there has been a total of 10 entrants.

Except for the 30+ times it has.
https://www.chess.com/tournament/live?search=Diamond&page=1
The main problem I see, is that you have to have a sufficient number of Diamond players on at the same time, in Live, and willing to play that specific time control. While also not being in a different Live tourney.
Edit... just realized I quoted a basic member that can't play in the events, so I guess it never would work
I've entered two so far and both were cancelled. The first one had four entrants and the second only two. Just wondered if this is the norm.