How do they all tech like crazy ???

[srad]

The truly remarkable space race games have victory pre 1500 AD

The fastest dates I have seen in xOTMs were in the 16xx. Before 1500AD is only possible with a low game speed (marathon, maybe epic), popping settlers, workers, techs from huts and capturing lots of cities in the beginning of the game (this means a really low difficult level).
E. g. getting a 17xx AD date in a normal monarch game is fast so don't bother about these "special setup games".

 

[srad]

So my question : is it only a matter of building more cities (I read about 24 cities when I had only a dozen), and if that is the fact, how do they pay for the maintainance, or is there some magic stuff involved ?

It's not only the number of cities but from my experience this is the key factor if you know how to handle the expansion. And after Currency you can build wealth so the limiting factor is mostly the missing room. So in 'normal' games (not like the scenario in the most recent BOTM) it is necessary to fight some early - mid game wars. I think late game wars will not help that much anymore.
If you look in the fast space race games you will often see that the civilization is close to the domination limit - and this is what makes space race also very interesting: For a competitive date you need to know how to fight a war and balance this with the research.

 

[iggymnrr]

The map often plays a big role is shaping strategies. Leader traits impact things to some extent also as does player skill level. One thing thats hard to beat is a large volume of cities. If a person can pay for them the sky is the limit.

You mentioned 12 cities. That can produce a 1850-1900 space colony win on a lot of maps with a number of techniques though the level of micro-managment can get tiring. Ultra fast space wins probably means you are able to choose a map and leader to fit like in an HoF setting.

Then, of course, there is the debate over corps or state property. Keep trying and it will surprise you what can be done.

As a note, a past gauntlet got me hooked on settler level (marathon) space games and offer up two different game styles: 490AD win with Darius (60+ cottages, 90+ windmills, state property) and a 630 AD win with Roosevelt (60+ cottages, 250+ windmills, cereal mills/mining inc, and malls.) If you were to guess that a cereal mills map was chosen for Roosevelt you would be correct. The tech that opens that corporation also opens up malls and is on the way to labs. There is more than one way to skin a cat.