For example if a surface/part instance has different stresses how can I group them according to percentage of elements with a stress of higher than for example 50mpa?
Probably, it is not the same that you want, but there is opportunity to paint these elements in desired color (for Plot Undeformed/Deformed Shape) using the Color Code dialog box: https://abaqus-docs.mit.edu/2017/English/SIMACAECAERefMap/simacae-c-ccocolornodeelem.htm
You need select Result value to specify elements containing results within desired range of values.