can I specify the ranges of the resolution shells in phenix.merging_statistics? (something like "XSCALE-word" RESOLUTION_SHELLS=)
I'm not familiar with XSCALE, but I think the answer is "no"; you can either specify the number of shells (over which all possible reflections in the resolution range will be evenly distributed), or you can specify high- and low-resolution cutoffs for the overall dataset.
and
how many resolution shells are ok? 10, 15, 20?
It's partly a matter of personal preference, but each shell should have enough unique reflections for the statistics to be meaningful. To be honest I'm not sure what this number is - I would guess at least a few hundred though. (I'm open to suggestions, since I'm trying to automatically estimate resolution cutoffs for phasing and refinement based on statistics in the outer shells.)