I have never, ever, on the biggest map, felt compelled or even interested in "having 20 cities", unless you mean puppeted capitols but liberty doesn't help that, except +1 happiness for all connected cities, and puppets don't get the additional 5%.
I play a game every other, if not every day, now consistently winning on immortal difficulty, random civ, standard or large size, epic pace, in all victory conditions. There's no reason, aside from your own, to have more than 9 non-puppeted cities, and very often I work with fewer. I'll have 4-7 by 500 AD and the rest by 1400, and everything else gets puppeted or burnt.
I know I'm rambling... what I'm really saying is "what you're portraying" as "the option" for the "best reason" for liberty "doesn't exist" because it "doesn't matter".
Ancient age. Yes, I always start there.