There’s really no such thing as “enough repeaters” in a Zigbee network 
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On a more serious note: the key to a healthy Zigbee mesh is redundancy, not just coverage. You want multiple routing paths so that if one router drops out, your devices can still reach the coordinator without issues.
A quick way to check is to open the Zigbee map in Z2M. If it looks like a tangled web of connections between repeaters, that’s actually a good sign; the more overlapping lines, the more resilient your network is. In Zigbee, messy is healthy.
Shameless plug: I don’t really fancy the Z2M map so I built my own.