With a conventional turbofan (or basically gas-anything) the bigger it is the more efficiency gains can be realised. So I'd expect that conventional jets would really ideally have just one engine, and they typically have 2 or 4 for reasons of fault tolerance and isolating noise from the fuselage.

None of this applies with electric, particularly a cleansheet design.

But then they would've been using many smaller ICE or jet engines already to get redundancy. I suspect the reason is that it's easier to get that kind of redundancy with electric motors - they're more compact, lighter weight, less overhead for each.

We do. It wasn't until the 80s that twin engine airliners were certified for long flights. Before then it was 3 or 4 engine only. It took a lot of effort to get jet engines that reliable too.

Single vs dual engine helicopter is a big safety distinction. There's heavy lift and military helicopters with 3 engines.