how many ultrasonic sensors can be connected at a time to UNO?
You can wire most ultrasonic sensor so you only need one pin. Assuming you are using Serial, that leave 18 pins you can use for ultrasonic sensors. With additional hardware, such as multiplexers and shift registers, you can attach a nearly unlimited quantity.
The NewPing library (Arduino Playground - NewPing Library) has an example sketch that supports 15 sensors using two pins each but they use the Arduino MEGA which has many more pins. On the UNO you could do that with 9 sensors using two pins each or 18 sensors using 1 pin each.