It is opt-in. If you're using Tokio then you can specify whether you want to use a single-threaded or multi-threaded runtime. Multi-threaded is "default" in the sense that if you just use `#[tokio::main]` then you get a multi-threaded runtime but you can also just do `#[tokio::main(flavor = "current_thread")]` to get a single threaded executor.

More to the point, even using a multi-threaded runtime won't spawn threads willy-nilly. It will default to using N worker threads (where N is the number of CPU cores available).