In my area, most shows have a rotation of about 4 - 12 singers, as soon as I hit 16 singers people will complain, people just aren't used to waiting more than an hour to sing (for most shows locally 45 minutes is an extreme wait).
A lot shows rely on passing trade and pre-existing patrons to the bar, so usually if you walk in, you will be singing in five minutes. People struggle to comprehend such a long wait when they aren't used to it, so I spend a lot of busy shows watching customers "tut", shrug shoulders and point to themselves in a "me me me" gesture if I'm not doing that I'm explaining to them why they cannot sing next. Lucklily for me I have a lot of regulars who understand and explain to the newbies why we all take turns and I won't skip rotation
A 13 singer rotation is just accetable, more so if people can see you calling up people quickly: a rotation which climbs, peaks at 16 and falls back is an easy night to handle (it rewards the early birds and night owls) but a constant 14+ from start to finish and people start stamping their feet.