Generally Merlin engines on the Falcon 9 are incredibly reliable. In over 200 flights there were only a couple of engine failures. Some worked nominally on accent but had some failure during landing.
> How many do they anticipate regularly?
The vast majority flights should never have an engine out. This is something that should generally never happen. I think SpaceX would hope that far less then 1% of flights have an engine out issue. Anything else I don't think they would consider acceptable.
SpaceX will redesign the Raptor engine as often as they have to to get the reliability to Merlin levels.
At least once they lost the booster when they were planning on recovering it because they had to run the ascent phase on only 8 engines and burned enough extra fuel the booster didn't have enough fuel to complete the boost back and landing.
I’m curious how many off engines the system tolerates. How many do they anticipate regularly? I’ve never noticed if Falcon 9s periodically run as a Falcon 8 or 7?
All and all, ‘‘twas an awesome sight. Kudos to all those cheering excited people.