Depth[expr] The depth of an expression is defined as one plus the maximum
number of Part indices required to reach any part of expr,
except for heads.
Depth[x]
Depth[x + y]
Depth[{{{{x}}}}]
Complex numbers are atomic, and hence have depth 1:
Depth[1 + 2 I]
Depth ignores heads:
Depth[f[a, b][c]]