I'm resolved on a single transporter system on the ship which is limited to only one transporter beam apparatus. The beam has power and circuitry for only six separate "frequencies" for beaming separate objects at the same time. Usually, one person is one object, but if two people are in contact or close proximity, then the system can beam them as one object. Since the transporter has only one beam and six frequency slots, it makes sense that only one transporter room with six transporter pads is needed at a time. The ship can have other transporter rooms (i.e. two to four), but due to the one beam, six frequency bottleneck of the transporter system, they would not be useable at the same time and would be sitting idle. When one room is in use, then perhaps to prevent unsafe interference from other rooms or something like that, then the other rooms are locked out of the system (like today's "lock out, tag out"). I am also in the camp that the transporter pads are a critical safety system and very maintenance intensive since molecular-level dematerializing/materializing is a precision operation, so, several rooms are needed in order to have one set of six pads certified "ready" at all times. Some sort of internal ship posting identifies which transporter room is ready for the shift. The "ready" transporter room becomes "the" transporter room in ship conversation to avoid confusion. Issue one: the changing appearance of the transporter room(s), namely food slots on the wall. Food slots were only seen in two episodes, four to five episode apart, namely Tomorrow is Yesterday and This Side of Paradise. Before, after and between these two episodes, the food slots were not there. It makes no sense in-universe to modify the room for food slots, then remove them only to put them back then remove them again ...Occam's Razor and KISS...there are more than one transporter room. Several other appearance examples come to mind further reinforcing the idea that there are more than one room versus continuously modifying this one room over and over again (like a TV stage set ). Issue two: Kirk disabled the transporter system at only one console. The Scalosians were on the ship for hours of ship time which to them could have been days modifying one room to beam them quicker. Deela implied that the transporter experience was slow. "DEELA: Yes. You beamed me aboard yourself when you came up. A ridiculously long process, but I've taken care of it." What's the problem to the Scalosians? Two ideas come to mind. One, as we discussed in the past, people in transit are conscious during the process. A couple of seconds seems to be okay for most humans (McCoy, Barclay and Hoshi are unsettled by the process though), but to the Scalosians, it would be hours of dematerialized consciousness which may be unbearable/maddening to do it a second time. Secondly, Deela's "fix" to the transporter system was needed to beam up the other Scalosians and their deep freeze equipment in a more timely manner in order to invaded the Enterprise and her many systems. Hours in transport would slow their timetable and give the Enterprise crew more time to stop them. In either case, she modified one room's console which was needed to operate that room's modified connection to the transporter system to make the beaming process faster. The Scalosians determined that fixing the sabotaged console would be faster than remodifying another room once they figured out where the problem was at. (Also, the Scalosians may not be able to get around the "lock out" or the maintenance "unreadiness" of the other rooms if those concepts are used. ) Issue three: the transporter system encompasses areas outside a transporter room. Most of the transporter operation was accessed at a control console in a transporter room, but not in all situations especially with the power/circuitry for the transporter. In The Enemy Within, the transporter control circuity was damaged in Engineering. The Doomsday Machine episode showed Scotty repairing the transporter/power system inside a Jefferies Tube. Scotty also accessed features of the transporter system inside the Emergency Manual Monitor Room in Mirror, Mirror (he added extra power from the engines and balanced it for the mass of four people) and The Lights of Zetar (mass beaming equipment directly into place). These examples give me the opinion that there are multiple transporter rooms, each with six dematerializing/materializing transporter pads with the limitation that only one transporter room (and its set of six pads) can be operated at a time, linked into one main control circuitry and one power system for the transporter beam. YMMV .