Interfacing large number of UARTs to PC
You could implement a multidrop line similar to RS485.
Perhaps you should read this.

RS485 is used in very long line industrial application and can be very useful. 
This EE-Times article lays it out quite well.

If you want simplicity, then you could implement separate send/receive lines, and use a very simple request/response protocol.
Operation at 115k baud should be possible, but it does depend on the quality of your twisted pair cable and the distances involved. 

If the sensors are simply sending data at an asynchronous rate then you'd need some means of throttling and synchronizing them.
You could:


Frontend the sensors with a small micro (say an Arduino Pro mini) that accumulates readings and either simply buffers the data or accumulates with time stamps if that is required. The micro responds to a software request/response protocol. Accumulating results and averaging could reduce the amount of data sent significantly.  
If your sensors support DTS/RTS, then an RS485 interface implemented via a small micro could simply stop readings. This time you use the software request/response protocol to halt sending at the sensor.