You have not given voltage detail of the lamp. 25 W is total energy it needs to light. But at what voltage ? Your answer will depend mainly on this information.
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Depends on the kind of panels you have available and what kind of voltage you need. With the right panel you can probably use 1. Or if all you have is a bunch of small cells from garden lights then you may need hundreds of them.
I know this sounds stupid but 25 watts worth!Voltage is your biggest problem.