using a Flux and flatMap will do all requests async, then you collectList and block to get the list of strings.

List<Strings> strings = Flux.fromIterable(dataList)
    .flatMap(data -> webclient.post( ... )
            .bodyValue(data)
            .retreive()
            .bodyToMono(String.class))
    .collectList()
    .block();

First of all, are you writing a Webflux application or just using WebClient in a spring web application instead of RestTemplate.

If using WebClient in a regular Spring web application you can just call Block() for WebClient to fetch the data block the thread until the data has been received and then return the data for you.

If using WebClient in a Webflux application you should return the Mono or Flux all the way out to the calling client, since it is the calling client that is the subscriber. The client (say a react app, angular app or whatever) subscribes to your spring application. Your spring application in turn subscribes to something else and will then just relay the data to the original subscriber.

You havn't stated what kind of application you are writing and you have a vague bit of code, which in turn means that the answer is also vague.

Yes it is required. The whole idea of being reactive is to make sure none of the Thread are blocked for IO.

You may have made your server side service reactive, but when your consuming that what is the benefit you get when your client is blocked untill there is a response from server. Your client thread keeps waiting untill the server responds. Which is not desired.

webClient.get()
        .uri("/persons/{id}", 42)
        .accept(MediaType.APPLICATION_JSON)
        .exchange().block()

will block your current client thread to wait till the server responds. This can block your client thread.

webClient.get()
        .uri("/persons/{id}", 42)
        .accept(MediaType.APPLICATION_JSON)
        .exchange()
        .then(response -> response.bodyToMono(Person.class));

Gives you a Mono which is a reference to a publisher that can emit a single value in future. So the client thread is non blocked.

I have blogged explaining this more. https://dzone.com/articles/spring-5-reactive-web-services