In all my use of Tokio in the last few years, I never heard of such a thing.
In tokio::fs::File [1] it calls spawn_mandatory_blocking to do file writes, I assume this is similar to spawn_blocking [2] which sends a task to Tokio's blocking thread pool. That thread pool is supposed to max out at 512 blocking threads [3], unrelated to CPU core count.
Tokio's TcpStream appears to be built on mio's TcpStream. I didn't dig deep into the code for this, but it doesn't just call spawn_blocking, and I'm assuming on Unix it ultimately registers the socket with epoll or equivalent, so it never blocks a thread to do socket reads or writes.
Could you share more about your application?
[1] https://docs.rs/tokio/latest/src/tokio/fs/file.rs.html#682
[2] https://docs.rs/tokio/latest/tokio/task/fn.spawn_blocking.ht...
[3] https://docs.rs/tokio/latest/tokio/runtime/struct.Builder.ht...