quantity

quantity /ˈkwɑːntəti/ noun
plural quantities
quantity
/ˈkwɑːntəti/
noun
plural quantities
Britannica Dictionary definition of QUANTITY
1 
: an amount or number of something
[count]
— often + of
[noncount]
— abbreviation qty.
2 
: a large amount or number of something
[count]
— + of
[noncount]

known quantity

: someone or something whose abilities or characteristics are well known

unknown quantity

: someone or something whose abilities or characteristics are not yet known