Arduino UNO Low-Power Consumption (Multiple Ultra-Sonic Sensors)
You did not specified, what kind of ultrasonic sensor you want to use so I assume use of "standard" ultrasonic Arduino modules (like hc-sr04), which need 4 pin (VCC, GND, TRIGGER and ECHO). Since Arduino Uno has got 20 IO pins (6 analog and 14 digital), you can use 10 pins as outputs for TRIG signals and 10 as inputs for ECHO signals and thus connect up to 10 ultrasonic modules to one Arduino Uno.  
In general, Arduino Uno is not designed for low-power applications. If you want more specific answer, you should ask more specific question...
Working temperature of Arduino Uno is -40 to 85 °C.
One of the options is using a terminal shield with screw terminals, e.g something like this.