If the resistance VR1 is too high the lamp may flicker or remain OFF, adjust the VR1 to adjust the brightness of the lamp. When supply voltage varies, you have to adjust potentiometer VR1 as stated above, for the proper performance of the circuit.
If you want to connect more bulbs, connect it serially and make sure that the current capacity of TRIAC and rectifier diode also withstands it. This circuit can be constructed with SCR also but mind the polarity to connect in the circuit. (SCR will conduct only one direction but TRIAC will conduct in both) IN4007 will give 1A output for the circuit. X1-1 and X1-2 are the terminal connection for connecting the light bulb serially with the circuit. Solder the components and enclose in a box to prevent electrocution. Fix the 1 Megaohm volume control pot on the box so that you can adjust the light through the safer side.