Hi, Depilego!
In this case, it is a PRONOUN, not a "number".
PRONOUN
Oxford gives you examples:
https://en.oxforddictionaries.com/definition/one
one
1 Referring to a person or thing previously mentioned or easily identified:
‘her mood changed from one of moroseness to one of joy’
‘her best apron, the white one’
‘do you want one?’
(That does not mean "Do you want 1?" but "do you want the one thing mentioned just before...")
Does that answer your question?