How can I get flag count of a flag?
For flag count on entities use: 

$flag_service = Drupal::service('flag.count');
$counts = $flag_service-&gt;getEntityFlagCounts($node);


For flag count on flag entity use:

$flag_service-&gt;getFlagFlaggingCount($flag);


You can find all functions from the Service here:

/modules/contrib/flag/src/FlagCountManager.php