1 exhaust fan for multiple rooms is a common design standard, depending on the room. If the comfort room is to operate with a light switch, then you're going to want a 1:1 room to fan ratio. If the exhaust fan is to operate on a schedule matching the building systems, then there shouldn't be anything wrong with a multiple:1 room to fan ratio.
I've designed an 40k square foot (convert to square meters on your own) building with 2 public restrooms, 1 private restroom, and a JC all connected to the building energy recovery ventilator. Since the exhaust is part of the building balance, this isn't a problem.
ZDR is not correct... I would say 90% of the time one exhaust fan is used to exhaust multiple spaces in a building. Very common practice and IS good design.
What is so comfortable in toilets? You have fashioned toilet seats?
One of elements that affects design decision here is usage. If your fan is working round the clock, there is no fear smell will transfer from one cubicle to another (if they are separated by full-height walls). You can install non-return dampers though.
The other element is noise transmission that can be important factor for high-luxury spaces, but yours do not belong to such it seems.
Now that we know you are exhausting toilets, the answer to your original question is "It Depends."
How do you intend to control the fan(s)? Do you need continuous exhaust or intermittent?
If the exhaust needs to be continuous, or at least operate only when the building is occupied or the ventilation system is on, then the answer is YES, a single exhaust fan makes sense.
However, if the toilets need only intermittent exhaust (on for only 15 min every hour), then one fan for each toilet is preferred. Not that you can't do one fan for both with intermittent exhaust, controlling them gets a little more complicated and you need to coordinate more with the electrical engineer.
If you put one fan you are limited on your control, you don't want your fan running whole day, that would compromise the 'economical' design. How would you know when people use the toilets as this is just and admin toilet? I could be wrong if this is a mall.
Why not run the fan all day? If taking the entire system into account, this is just less airflow on the return fan.
That being said, if there's one fixture and 70 cfm exhaust, just put simple on/off controls. Than you can utilize a switch, or occupancy sensor with a timer. Intermittent use shouldn't have too great of an impact on the overall system.
There are literally dozens of ways to handle this situation, and because looking at it from the outside doesn't seem "economical" doesn't mean it can't be part of the larger solution and actually improve the efficiency of the system.
An energy savings trick I leaned from a US Congressman, as installed in a US airport bathroom:
Place the lighting and ventilation on a motion sensor on the outside of the stall. The only way to keep ventilation and light on is to adopt a "wide stance" and waive your feet and hands under the stall. People coming into a dimly lit bathroom with people waving their hands and feet under the stalls may be enough to change the mind of potential occupants and use a more conventional setting. Saves electricity and conditioned air.
