Hi. Here's my experience -- I hope it helps.
I experienced a lot of frustration trying to find enough habitable planets in my early games. I found the default settings made habitable planets too rare. So...when setting up galaxy options for a new game, I would increase both "Planet Frequency" and "Habitable Planet Frequency" by one step until I found something I was happy with.
I like Large galaxies, and so I settled on Planet Frequency=Abundant, Habitable Planet Frequency=Uncommon. It's all personal preference and depends also on how many opponents you have. For me and six opponents, this was good to have enough worlds to play with, but not so many that the game would become a management nightmare. Here's my setup for a large galaxy with six opponents: https://www.dropbox.com/s/9gqr8jbyj2rewyt/galciv3_large_galaxy_settings.jpg?dl=0
As for how the Altarians got four colonies.... Some races start with a single class 16 homeworld, but others, like the Altarians, get a smaller class 10 homeworld with a small class 6 neighbour that's easy to colonize quickly. So that's two planets for them. You can rush buy at least a couple of colonizers, and they probably got lucky finding nearby habitable planets (there is enough randomization that it's easier for some to get a quicker start than others).
I find the AI builds quite a few early scouts. They're cheap to build and cheap to buy. This gives them a larger number of ships, but they're completely harmless (no weapons). More scouts means you can find habitable planets sooner, I guess. I prefer to just build colonizers in the beginning, but I have to be extra careful not to run into pirates when hunting for new worlds (losing a scout to pirates isn't too bad, but loosing colonists to pirates really hurts).
The first two techs I always research are the early speed-boosting Engineering techs: Interstellar Travel and Hyperdrive Specialization. These give all your ships +2 moves which really helps your colonizers and scouts get to those habitable planets quicker.
erischild's advice above to get the first Pragmatic Ideology trait "Builders" and then upgrade the free constructors to colonizers is really good. However, I usually retain one of the constructors to build my first economic starbase near my homeworld.