Consider the various GC3 win-conditions.

• Conquest.  Your ships beat everybody else's ships.  Your mobile pop (troops) beat their immobile pop (civs).  This probably requires a larger production base.  Having more planets is probably necessary.  Hence the mainline strategy is: get planets, build them up, amass fleets, fight, invade.
• Cultural.  Own 75%+ of the galaxy within your ZOCs, and not be at war with anybody.  This emphasizes the total influence clock, which is disjoint from pop and military.  It still helps greatly to own many planets tho, just to have your ZOC clocks radiate outward from many centers.  You'll need enough defense to, um, not get invaded by all the conquest-victory sheep.
• Diplomatic.  Ally everybody.  You might not need planets at all for this.  OTOH, if UP votes are proportional to pop or ZOC, then ... see above, do above (at least a little bit).  Some diehards will never ally, so arrange for somebody to overrun them (maybe even you).
• Technological.  Research enough Age of Ascension techs, and you Just Win.  Expect the rest of the galaxy to unite against you to stop it.  However, tech is best researched by having many, many researchers.  So planets helps.
• Ascension(?).

All of these strategies benefit from owning more people.  All of them probably can be confounded by an enemy who owns more good planets than you do.  Such a foe will either out-do you head-to-head in your own victory-race, or undercut you with another victory-path and just kill you off first.  (Corollary: Whatever path you take, along that path to success you will surely beat many competitors, regardless of the path they were pursuing.  So it cuts both ways.)

Now, all strategies probably have ways of not caring whether you colonize planets first.  So pure racing might not be that important.

• At one extreme, you swarm nearby space with nigh-empty colonizers, to grab planets with 0.2 - 0.5 bp each.  They fly your flag, but may produce nothing useful for 10-25 turns of growth.  (Funny: if growth were exponential, they'd take even longer!!)
• At the other extreme, move your shipyard 2 hexes SE of your pop world, so that new colonizers pop out adjacent to the planet.  Immediately land on planet, take off and set pop slider to max 5.0 bp (which is double the default).  The extra +2.5 bp saves your colony 16.7 turns(!) of growth at the default 0.10 bp/turn + 50% Approval bonus.  Then your colonies might be 16 turns closer to completing a full hex ring with Technological Capital in the middle.  This could win big if your 1st nearby planet is much better than your homeworld.
• In the middle, just send colonizers with the default half-full load of 2.5 bp.  You'll out-produce any nearby nigh-empty enemy colonies, so you can outrace them to shipyard + weenie and dominate their space, or outrace them to Consulate and flip them, or maybe outrace them to UP votes.  You'll out-number the double-the-default colonies, so maybe you can punch through their turtle-like shells before they Just Win.

We are all hoping that GC3 balances the various victory-paths to allow many strategies to flourish and be equally deadly.  In any case, a serious win will almost surely require a mix of several victory-path snippets.  As a general rule, though, you probably need close to 1/n of the map's planets, where n is the number of players, to be competitive.

That's pretty much why.  The game has only one option and state and it's victory through constant war.  Beta 2 will introduce two more victory conditions at least and the game will no longer start with everyone at war.  It should mean you don't have to be as aggressively expansionary to get a win.

Thanks for all the thoughts and inputs. I thought abit more about it, and it think what is "missing" for "one planet strategy" to be viable, is some kind of global penalization for having more planets. In Civ this was happiness. The only "penalizing" for taking another planet, is the manufactoring and population you put into the initial build of the colony ship, which will be given back over time.

Happiness puts a cap on how many cities you can have with a certain population on a gloabal level, there is no such "resource" in GalCiv?

In GC2 it was "morale" which was affected by planet quality/tax rate and overcrowding which could be increased by building entertainment centres and building mining starbases on morale resources (yellow cylinders) in space assuming you find and beat everyone else there with a constructor first as the minor races tend to make a beeline for them! oh and certain trade goods helped!

hope this helps? But I dont know how Brad and Paul are gonna do this in GC3 as it hasnt been included yet as far as I know?

I haven't played Civ in a while but I believe there is not a hard limit on how powerful one city can become. As the Admiral said in Galciv, your home planet is a PQ10 and that makes it hard to win with the one planet.

I seem to recall a few trying this strategy in GC2 but I don't recall any results. I don't see any way to make it work.

Couple of things:

In RTS games, there's hard limits for UnitCaps, or the like. This means that Turtling is a valid strategy, because you can always pretty much match your opponent's total force level, plus you have defensive structures. By turtling, you can force an opponent to burn through their resource advantage quickly, since you typically require 3:1 force advantage to win against a defending force.

In an open game, where there are no resource limits or unit limits or any such artificial barrier, more is ALWAYS better than less. Even morale can't be a limit, because if you have sufficient money (which, larger populations produce more of), you can always bribe people to stay happy.

In the end, it's all about economic power, regardless of the actual "win condition". Access to large quantities of money is what enables ALL the other win scenarios, and the only way to generate that kind of cash is via a large population, which in turn requires large numbers of worlds.

There's a reason why the USA is many times stronger than Singapore, by any measure of "strength".

There's some game mechanics to be discussed as to whether it would be better to have a modest number of high-population worlds (which you presumably can have economic improvements make the most benefit from) or a larger number of worlds which have much less per-world population but have much more room for a large total number of economic improvements.

i.e. which is better:

1) 10 worlds, each with 10 billion people, and room for 50 economic improvements.

2) 25 worlds, each with 4 billion people, and room for 100 economic improvements.

Remember, economic activity isn't (or shouldn't be) linear, so a world with 10 billion people on it should produce MUCH more money than 10x a world with 1 billion people on it.  How much this multiplier is, is game balance, but economic revenue should definitely be geometric, not linear, to population.