airdrop.min_players
cargoship.loot_rounds​

You could also probably set up a script (there may be a plugin to handle this kind of thing as well) to modify cargoship.event_enabled from False to True when the desired player count threshold has been met, or just change it manually at will, or set cargoship.loot_rounds "0" so it only spawns 1 crate.

FancyDrop is a powerful plugin, just overkill if all you want is to add minimum player cap before airdrops.

A less invasive way would be to override OnEventTrigger and filtering on the prefab guid for CargoPlane.
Then based on your return result, you can control to either continue the event or to end it before it begins (based on your own criteria).

Example:

private object OnEventTrigger(TriggeredEventPrefab triggeredEvent)
{
    // Should not happen
    if(triggeredEvent.targetPrefab == null)
    {                
        // Do nothing
        return null;
    }
   
    // If this is not an airdrop (i.e. Helicopter, CH47) then return null (meaning continue normal trigger)
    if (!triggeredEvent.targetPrefab.guid.Equals("8429b072581d64747bfe17eab7852b42"))
    {
        Puts($"{triggeredEvent.targetPrefab.resourcePath} event allowed to continue [{triggeredEvent.targetPrefab.guid}]");
        return null;
    }

    // Just an example filter - check if player count is above hardcoded 10 players     
    if (BasePlayer.activePlayerList.Count() >= 10)
    {
        return null;
    }

    // Report to console trigger event was blocked
    Puts($"{triggeredEvent.targetPrefab.resourcePath} blocked");

    // Returning false will stop triggered event!
    return false;
}