number cruncher vs reckoner

number cruncher and reckoner both are nouns.

Word	Noun	Adjective	Verb	Adverb
number cruncher	Yes	No	No	No
reckoner	Yes	No	No	No

As nouns, reckoner is a hypernym of number cruncher; that is, reckoner is a word with a broader meaning than number cruncher:

Other hypernyms of number cruncher include calculator, computer, estimator, figurer.

number cruncher (noun)	reckoner (noun)
a computer capable of performing a large number of mathematical operations per second	a handbook of tables used to facilitate computation
someone able to perform complex and lengthy calculations	an expert at calculation (or at operating calculating machines)