In most locations a bunch of 6 or extra has a compulsory service cost, that is the tip.
In the event you have been getting nice service and there have been a number of servers in your specific case extra tip can be thoughtful.
As an individual who eats out lots, I might think about the service can be horrible if one particular person was serving 20+ individuals. 20% is perhaps ample, relying on how good or unhealthy it was.
If it was good I most likely would have left a bit extra, however that is simply me I would not count on anybody else to do it.