One can estimate limits by considering the recognized
maximum number of parameters refinable by the Rietveld method.
For minimal FWHM of the order of 0.12° 2-theta, 50 to 70 free atomic coordinates (x,y,z) are refinable reasonably .
This corresponds to 17 up to 23 independent atoms in general position .
Corresponding to these limits, cell volumes can be more or less estimated, depending on the crystal system and on the Bravais lattice. For centrosymmetrical space groups, these maximal volumes are the following :
Vmax(Å3) Multiplicity of the Lattice System general position
500 2 P Triclinic 1000 4 P Monoclinic 2000 8 C ,, 2000 8 P Orthorhombic 4000 16 A,B,C,I ,, 8000 32 F ,, etc for tetragonal, hexagonal and trigonal 12000 48 P Cubic 24000 96 I Cubic 48000 192 F Cubic
One will have approximately 20 reflections per xyz refined parameter.
In a single crystal study, 10 reflections per parameter, including the thermal ones, is something considered as normal.
The fact that a larger value is proposed for powder data is a consequence of reflection overlapping.