... I wonder how many lines of code the Plan9 (C) server takes? (much less I suspect)
If the C ends up hand rolling something Rust just has built-in, that adds up pretty quickly. For example Rust's arrays, strings etc. know how big they are, C's do not and so you need extra code to re-calculate or to explicitly store and retrieve those sizes. On the other hand it may be tempting to express a Rust type as having methods defined over it, rather than as you must in C using only free functions throughout, there's also a Rust discipline of always deriving common traits e.g. Clone, Debug and Eq, when they're appropriate, as a convenience, that's one line extra in typical style.
You can think of the Plan9 kernel as a 9P multiplexer. 9P in, 9P out. The closest matching thing would be e.g. a 9P-to-ext4-in-a-file translator -- but it's always "9P server talking to $FOO" (or "9P server that makes files up on the fly").
Most of the complexity of this Linux 9P server would be in translating between the unrelated worlds of 9P and Linux VFS. A more apt comparison is the size of this 9P server vs an in-kernel NFS server -- they both translate something unrelated to Linux VFS.