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I'd suppose there would be the same percentage of priests hoping to be pope someday, as 5 year old boys wishing to be firemen.
Realistically speaking... there's only one Pope at a time, out of the hundreds of thousands of priests worldwide, and that pope is elected by hundreds of bishops and cardinals... not to mention that his Papacy is likely to extend a few decades...
The likelihood of your average priest ever becoming Pope is extremely low, therefore I think if a young man decided to become a priest hoping to be Pope someday, he'd very likely be advised against ordination in the seminarial stage.