When selecting your mode of transportation for the day of your dreams, take my advice...pessimism rules.
It's been my experience that what is advertised as a 8-10 passenger limo, typically comfortably seats 8 adults, if that much. Logically, it makes sense. Bridal party participants vary in height, size, weight etc. so if you're trying to seat 10 women as opposed to 10 men, you may have different challenges.
Here's my advice, in addition to actually going to SEE the vehicles maintained by your limousine company of choice, when in doubt, super size. If you have 11 adults in your bridal party, unless they are all very slim, consider reserving a shuttle or limobus as opposed to a "stretch" 10-12 passenger car.
Not only are shuttles comfortable, these days the style options are endless and you're guarenteed 10 individual seats as opposed to 1 long winding seat and a prayer.