Multiseat events are just like multiprice events. But where you create price categories (or price levels) with multiprice events, here you can define (if wanted) the maximum available seats per price category.
This means that if you define multiseats, the number of seat “categories” must correspond with the number of price categories (even if the price is the same).
You can also still just define a “single” seat maximum for multiprice events, which means what it has always meant: the maximum number of seats, even if you define multipe price categories.
A screenshot for multiseat categories (3 seat categories: 10 seats at €7 each, 20 seats at €6 each, 25 seats at €5 each):
Don’t forget to also adapt your RSVP form to include #_SEATS{1}, #_SEATS{2} etc … (see the doc on multiprice events)