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What's the voltage and current rating of the LED you're using?
The voltage will determine how many cells you need, and the current times the backup time will determine the Ah/mAh rating of the battery.
So, for example, if you are using a 4 volt LED that draws 300 mA, using 1.2V NiMH cells you would need 3.33 cells (make it 4) and to get 10 hours you'd need a 3000 mAh capacity. (C sized cells).
Seems like a lot for a solar garden light... most solar garden accent lights are a fraction of the wattage, usually 1-2 miniature white LEDs that draw 1/10th the power. The solar panel is going to have to be several inches square to produce enough power to charge the batteries.