That's an interesting math problem. There are 8 seats per ride side. Let's say that people with the same letter are friends....
AABBCCDD, EEFFGGHH (8 pairs)
AAABBCCC, DDEEFFGG (2 trios, and 5 pairs)
AAAABBCC, DDEEFFGG (1 quartet, 6 pairs)
AAABBBCC, DDDEEFFG (2 trios, 5 pairs, 1 single rider)... for this example, he could ask for a trio instead of riders F, F, and G.
I guess a motivated ride operator could fill all the seats all the time if he asks for the number of people he needs. For example, if he needs a trio, but there are no trios at the front of the line, he could ask for a trio that is 30 people back on the line. The most common groups are probably pairs and trios, which makes it easier to avoid needing a single rider.
The fewer seats a car has, the better it is for single riders. Twisted Colossus will always need single riders to sit with the third wheel of a trio. Radiator Springs Racers has 3 seats across and will need single riders because there are lots of pairs. Drop of Doom is a nightmare for single riders IF the ride operator knows what he is doing.