Small World our party of 20 had to split into 2 boats. i think 12 will fit in one depending on sizes (4 rows of 3?).
POTC we had a boat to ourselves and an empty row in the back.
BTMRR is harder to say, because a row can fit 2-3 people depending on sizes. I'm not sure how many rows there are in a train, 10-12?
6-7 in a Splash log (7 if a small child and adult ride in the back row).
8 in a Space mt car, TSMM car.....
Aprox 8 per row for Star Tours