You are correct. Statiscally you need to invite three times as many men who you hope to actually show up. When we did our gangbang party, my husband (who is very organized and thorough) spent a lot of time vetting and verifying the men we invited.Always book 30 so 10 show up. So many no showers and talkers with no game or the the guy who shows up and is done in 5 minutes.
I think we had about 22-23 men initially planning to attend. Over the course of about a week before, we had several cancel or drop out for one reason or another. We were down to about 13-14 a day or two before the party, had a few cancel the day of and a few more just not show up. We ended up with 8 black men actually attend in addition to the three wives and three husbands.