Commentary Par XXIX 130-132

The numbers of the angels increases the higher they are found.  Dante's coinage, the verb ingradarsi, is found in slightly altered form (digradarsi) in [Par XXX 125].  In both cases it seems to have the meaning 'to increase step-by-step.'  Thus, the higher the eyes of an observer mount, the more angels they are able to take in.  And that number is both beyond human vocabulary and mortal conception.