This is not a flag you need to enable in chrome://flags for most users; it has been gradually enabled via Google’s backend. The term "mostvisited9" has become an internal shorthand in Chromium discussions, representing the new default shortcut capacity.
The new update makes manual editing easier than ever. You no longer need third-party extensions. chrome newtab mostvisited9 updated
If you only see 8 tiles, try the following: This is not a flag you need to