2 lenses.
Nightscapes require wide lenses. But, they also have to be fast (f/2.8 or faster).
As the 60Da is a crop (1.6x) sensor, I'm not sure which Canon lens would be wide enough for nightscapes. Perhaps the 10-22mm, which is approximately equivalent to the 17-40mm on the 35mm sensor.
The 24-105mm f/4L IS USM can't be beat for a general purpose lens.
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