number system vs system of numeration

number system and system of numeration both are nouns.

Word	Noun	Adjective	Verb	Adverb
number system	Yes	No	No	No
system of numeration	Yes	No	No	No

As nouns, number system and system of numeration are synonyms defined as:

number system and system of numeration: any notation for the representation of numbers

Other synonyms of number system include number representation system, numeration system.

number system (noun)	system of numeration (noun)
any notation for the representation of numbers	any notation for the representation of numbers