The line between 100 and 500 contains a lot of people (5)
I believe the answer is:
crowd
'a lot of people' is the definition.
I can't tell whether this defines the answer.
'the line between 100 and 500' is the wordplay.
'the line' becomes 'row' (a row or line of things).
'between' is an insertion indicator.
'100' becomes 'c' (Roman numeral).
'and' says to put letters next to each other.
'500' becomes 'D' (Roman numeral for 500).
'c'+'d'='cd'
'row' put inside 'cd' is 'CROWD'.
'contains' is the link.
Can you help me to learn more?
(Other definitions for crowd that I've seen before include "Flock" , "Rabble" , "Spectators" , "Multitude" , "Large group of people" .)