Sorry, but I don't get it. What is the signal line? Do you mean the clock line?
No, any digital IO of the Arduino. As an example, you can use D3 for this but I don't know if this pin is unused in your design yet.
If I can synchronise different arduinos , I'll be using the same board : arduino uno.
That's fine but keep in mind that the UNO is not the most accurate of all types in terms of timing.
The number is not fixed there can be many sensors (up to 30)
But it would be fine if I can simulate from 10 to 20 sensors.
In that number I would choose cheaper variants than a complete UNO for every sensor but that's up to you to decide.