I bet they're less concerned with # of firings and more with fatigue/wear from atmosphere reentry. Very hard to test on ground.
They've now got 45 intact engines back, from a variety of re-entry regimes. We know they've done an all-up static fire. Several of the returned stages have had engines removed and I imagine some of these have been tested.
If there's a major problem, they already know about it.
But I'm pretty convinced they're trying to avoid this. The engines have been test-fired many, many, times (30 starts is a figure that has been mentioned a few times. Remember that the first recovered stage (Orbcom) was returned to the launch pad and went through a static fire just a few weeks after landing. It seems clear to me that they've got their eye on reasonably-quick-turnaround reuse.