sentence - Refer to a number which comes after every 5 numbers

I think you're after a single word like quinquennial, which refers to “every fifth year”.

I looked up the phase “every fifth” in OED. There are five definitions which include it, one of which is irrelevant and another which is duplicated.

penteteric, occurring every fifth year (from Greek)
quinquennial, occurring every fifth year (from Latin)
quintan, occurring every fifth day

Those are specific. There isn't a word for every fifth item in a general sequence.

Thus for an adverb similar to quinquennially you would need an adverbial phrase such as at or after every fifth number.