Yes they are limited, it is first come - first served.
Simply keep in mind, while using Supersaver tickets, you are giving up flexibility, while the latter is, when travelling as a group, often needed.
Group tickets are strongly recommended as they offer more benefits such as free of charge seat reservations. To find seats together for a party of 10 can, depending on the route and travel time, be a challenging task. Furthermore, group tickets can be alterated post-paid in case of last-minute changes due to fewer or more group members, or change of travel plans, which is all not possible with Saver tickets. Also, if you miss the connection because some of your group members couldn’t make the connection as according to the timetable (walking issues, medical issues, being lost somewhere in the shops, etc.), your tickets are no longer valid for a later connection.
If you gonna buy Saver tickets still, each ticket must bear the exact names of the person travelling and everyone has to carry the passport or official ID card physically with them. Both is not needed with a group ticket.