reactor-core/src/main/java/reactor/core/publisher/Mono.java
public final <R> Flux<R> flatMapMany(Function<? super T, ? extends Publisher<? extends R>> mapper) {
		return Flux.onAssembly(new MonoFlatMapMany<>(this, mapper));
	}
public final <R> Flux<R> flatMapMany(Function<? super T, ? extends Publisher<? extends R>> mapper) {
		return Flux.onAssembly(new MonoFlatMapMany<>(this, mapper));
	}
Transform the item emitted by this {@link Mono} into a Publisher, then forward
its emissions into the returned {@link Flux}.

<p>
<img class="marble" src="doc-files/marbles/flatMapMany.svg" alt="">

@param mapper the
{@link Function} to produce a sequence of R from the the eventual passed {@link Subscriber#onNext}
@param <R> the merged sequence type

@return a new {@link Flux} as the sequence is not guaranteed to be single at most