It sounds like (unsurprisingly) Singapore uses the same system used here in Hong Kong. I'm not sure if the same system is used in mainland China.
fist = 0
index = 1
index, middle = 2
index, middle, ring = 3
index, middle, ring, pinky = 4
index, middle, ring, pinky, thumb splayed = 5
thumb, pinky = 6
index, middle, ring, pinky, thumb all held strait with their pads touching, looking a bit like a shadow puppet of a bird's head = 7
thumb, index = 8
just the index finger, bent in half = 9
Apart from 4 and 5, you can kind of see the link between the written form of the number and the hand signal:
1, 2, 3: obvious because 一二三
6: the thumb and pinky make up the bottom part of the character 六
7: If you look at your own right hand upside-down when making the gesture, the bird's head points in the same direction as the 七
8: Your thumb and index finger are the two lines in 八
9: Your index finger is the second stroke in 九
I don't know whether this is coincidental, or even just my imagination...
fist = 0 index = 1 index, middle = 2 index, middle, ring = 3 index, middle, ring, pinky = 4 index, middle, ring, pinky, thumb splayed = 5 thumb, pinky = 6 index, middle, ring, pinky, thumb all held strait with their pads touching, looking a bit like a shadow puppet of a bird's head = 7 thumb, index = 8 just the index finger, bent in half = 9