The bolts are m6 x 25 long there are 8 of them fastening 4 seat fixing saddles, so a 10 mm AF socket spanner fits,
@Sestina is correct, there are captive nuts in the floor, though depending on how they have been treated the threads could be damaged, I always apply a bit of lube to the threads when replacing the seats after having them out.
Each seat assembly has or should have a couple of saddle type devices with plastic bushes on the bottom bar that the bolts go through to hold the seats in place, the 2/3 width seat mounts in the RH side the 1/3 seat mounts to the LH side, the seat belts still may be in place but if not they can be a bit confusing to attach correctly or in the correct buckle positions.