There is no magic about 120 radials, and long before 120 radials are
reached the increase in field strength pretty much stops.
According to the 1937 BL&E experiments and IRE paper, this depends on their
If they are short (~0.15WL), then there is little improvement in radiated
field when going from 30 to 120 buried radials. But that is not true when
they are long (~0.4WL).
This is nicely summarized in the clip below from a 1947 IRE paper by Carl E
Smith, a well-known broadcast consulting engineer.
Topband reflector - firstname.lastname@example.org