yes, getting out second city is very important. as soon as you hit 2 pop capital, as people said above. with few exceptions.
but i do not understand, why anyone needs 20!!! cities? why, why? for what kind of scenario 20 cities are needed? or how this is enjoyable? all that work with usually bad, endgame cities is mostly at the time of game, where player is near the end. game is already a chore often at that moment. why delay win with even more clickwork of these nearly useless to win cities?
6 cities is all you need on normal map for any victory. and it is not even "need". i play mostly large maps and rarely go over 9-10 cities. well, there are sometimes resources needed, ok. or interesting natural wonder. problem is, these late eras cities suffer from immense costs of building someting meaningful, beyond city center infrastructure.
as for starting.
if you have improvable resource and no threat, go for worker, research corresponding tech.
if not, go for scout or slinger.
about starting tech.
listen, if you play on prince, you do not need even single campus in your whole game to lead in tech, see? id say, instead of writing go for something what will give you production and firepower - animal husbandry, masonry or sailing or certain map, that way you can explore seas with 1 charge worker very early.