If you talking about a restaurant, you only need one reservation for a group - You make one reservation for a table.
In a train, it would be more common for each person to have a reservation. One reservation for each seat.
"I have reservations about how much to expect from posters." This would be more natural than "have a reservation".
Restaurant Patron: May we have a table for four?
Waiter: Do you have reservations?
Patron: Several, but we thought we'd try your place anyway.
- For Teachers