I require additional digital input pins for 5 PIR sensors. Can I use the shift register (74HC165) to read these input? And will I be able obtain real time values from all the 5 sensors?
You could use a GPIO expander (MCP23017 or similar) which could give you 14 extra pins (16-2 for I2C bus), but this is putting extra load on the microprocessor. (you haven't said which one by the way).
Or another option might be to revisit you design and see if you can split it out to multiple processors. This is effectively the same as a GPIO Expander, just using an Uno or ESP8266 as the expander.