I search the index for "max", eventually I get to: <https://nim-lang.org/docs/theindex.html#INT_MAX>
which leads me to: <https://nim-lang.org/docs/system.html#int-max_1> "high(int) is Nim's way of writing INT_MAX or MAX_INT." Then I use a fraction of my brain's activity and try out `echo high(uint64)`. And it works.
