In 2021 Thunderbolt 4 docks are now available, which for the first time allow for USB-C / Thunderbolt true multi-port hubs.
For the first time, Thunderbolt 4 will offer docks with up to four Thunderbolt ports and universal cables up to 2 meters in length. – Intel press release
These hubs also seem to pass through USB-C DisplayPort Alt Mode signals, allowing them to be used with non-Thunderbolt devices.
Thunderbolt 4 hubs:
Simple one-to-many USB-C hubs with a large number of ports, for some reason, do not really seem to exist.
There are some 2 and 4-port USB-C hubs now available, but none seem to be cheap, simple, or reliable. It's very easy to find 7-port USB-A 3.0 hubs from well-known companies; the same cannot be said for USB-C. There appears to be some kind of spec, chip, or marketing limitation the prevents them from being made.
Some of the same complaints are outlined here, from when zero port-duplicating "hubs" were available:
None of these USB-C “hubs” actually give you more USB-C ports.
There is supposedly news for 2019/2020 that suggests hub chips may be coming soon:
Update (2018-07-30): Accidental Tech Podcast reports on a rumor that next year Intel will finally ship the chip that’s needed for making a USB-C that adds additional type-C ports.
We've heard from an anonymous person that Intel is finally releasing a hub chip for USB-C next year. We may not see products based on it until 2020.