As students enter the room, assign each a number (use consecutive numbers rather than random numbers, but they can be given out in random order rather than in numerical order around the room)
Have a run through - students count as a class by calling out their number at the appropriate moment. See how fast the class can get through it (with correct pronunciation of course!). Explain that some time during the lesson you will call out a key word (like "number off!" or "Ada berapa?") without giving any other prompting. At that time they will need to count again - good to see who is listening and paying attention to what is happening in the class! You might like to assign yourself the number 1, or choose to give the number 1 to someone reliable and / or near the front so that it won't totally fall flat.
Good for drilling numbers when first learning them.