You have some guidelines in my old answer "ClearCase to Git migration" (2014), but more generally, I would add (2016):
ClearCase being centralized, there is no limit in size for a Vob or even an UCM component (be it an UCM Vob, or a top folder within an UCM Vob).
So while the rule of thumb is one git repo per UCM component, it needs to be checked against the actual characteristic of said component.
Since a git repo represents an history of "snapshots" (not the ClearCase snapshots, but more like ClearCase tags), I only import ClearCase baselines into a git repo, not intermediate versions per files. The baseline applies to all files in an UCM component, which is more inline with git commits.