Most people assume when they have 10 people in their party they need a 10 passenger limo. The industry standard rates the capacity of limousines for maximum capacity. That does not always mean comfort however. The rule of thumb is subtract 2 for comfort. In other words 8 is a comfortable amount of people in a 10 passenger limo.
This is especially true for more formal occasions when evening gowns, bridal dresses and prom dresses are a factor in the equation.