50 is a good number of people, especially if you're doing sample sales i.e. 1pm - 4pm special sales in a secret location by XX designer. For instance you can whatsapp a group of well-known fashion bloggers, who may forward the invite to a group of friends etc etc creating a semi-exclusive shopping experience

I've seen people do it for yard sales too (what we call void deck sales) It's a bit like a tupperware party.