According to the 2020 Rust Survey[0], a decent amount. The majority is on stable rust, but a bit less than a third of rust users are on nightly.
The company I work at uses a pinned nightly rust in order to access some soon-to-be-stabilized features that simplify our life a bit. We update our pinned nightly in lockstep with stable rust releases. In practice, we've almost never had problems with using nightly rust - those are still very well tested, and problems are caught early and fast. The Rust test suite is quite thorough!
However, we do try to avoid using features that don't have a clear trajectory towards stabilization, so for us to consider thread_locals, we'd need to have some very solid proof that it would fix some critical performance problems.
I suspect every org will have their own policy, and while the majority will use stable rust, it's not like nightly rust is completely unthinkable.
This survey says about half of the responders are using Rust in production (or at work in some capacity). It doesn't say anything about what percentage of those surveyed AND use Rust at work are ALSO using stable vs nightly.
