Hi Guy,
It's in the Notation Reference (NR) under Rehearsal Marks.
\set Score.markFormatter = #format-mark-circle-numbers
Would that be good enough? If not, just use a \markup, but then you have to manage your own numbers.
The NR is dense and large, but it is worth reading it over and over again.
Andrew
Dear Valentin,
thanks so much for your quick and right-to-the-point answer. Thanks for suggesting to group both the mark and number reset into a single command. I'll have to dig a bit to see how I can have the mark wrapped into a rounded box (replacing the \default command using something that invokes \rounded-box \bold ?).