How many wires can be connected to the C terminal at the furnace?
Technically, there should only be one wire connected to the screw terminal. However, it's common practice in the HVAC industry to connect more. 

If you want to do it the proper way, you should use a twist-on wire connector (or other connector), and a length of scrap wire to make a pigtail.  


Disconnect the wires that are currently attached to the terminal.
Group together those two wires with the wire you want to add, and a short bit of scrap wire of about the same gauge.
Use the twist-on wire connector (or other connector) to connect all the wires together.
Attach the other end of the scrap piece of wire to the screw terminal.




You may also want to consider why there are already two wires. It's common in a furnace to have the A/C contactor coil terminated there, but you'll want to figure out where the other wire goes.  The concern here, is that you could overload the transformer. If the transformer is operating at or near maximum capacity, the additional load of the thermostat could push it beyond the limit.  Best case here would be a blown fuse, worst case being a blown transformer.

If the transformer is powering a humidifier or air cleaner, you'll want to make sure it can handle the additional load of the thermostat (though thermostats are typically a small load).