If your C4D licenses issued from a license server, or you have floating licenses, you need to purchase the same number of licenses as your server can issue.
you have 100 seats
you have 10 floating licenses
you should purchase 4 multi licenses. for 4 x 3 seats
you have 10 seats
you have 2 floating licenses
you should purchase 1 multi licenses. for 1 x 3 seats
you have 20 seats
you have 5 floating licenses
you should purchase 2 multi licenses. for 2 x 3 seats
if you need lots of seats, ask support for a volume price.
NOTE : A single user license, will not run on a floating seat.
NOTE : If you have a render farm, there is a branded site license available for each plugin. This costs more as it is compiled and branded for your company, but is very easy to install on lots of clients, and has no restrictions.