Looks like you have 3 strings of 6 LEDs.
You should be able to verify this by following the traces on the board, there will be three sets of 6 LEDs wired + to - with the beginning and end of each set connected to the + and - busses along the top and bottom of the board.
The fact that only three LEDS are lit is perplexing - was this light working before and recently failed? There may be multiple failures we are dealing with here.
I would measure resistance across the + and - busses - so from the DC+ on one end to the DC+ on the other end, and the same for DC-. That way you can determine if either bus is damaged.
Then I would measure resistance across each diode individually, looking for anomalies.
edit: in case it is not obvious, test resistance with the light unplugged