Q:

What reasons are there for using an interstellar ship that can only carry 8 people to settle another planet?

In my story a civilization has reached the point where they want to colonize another planet. They encounter a problem. Their spaceship can only carry a crew of eight (8) people. 
Specifics of the journey:
-The voyage takes approximately seven (7) years and covers a distance of 2.3 light years.
-The technology available regarding life support and computing systems are comparable to current day modern tech.
-People cannot be kept in stasis for any amount of time.
-The civilization has access to "magetech" which is an alternate evolution of technology in conjunction with magic. But this technology is very expensive and scarce due to its resource requirements.
-People who wield magic, can only generate energy. Water/food or conjuration of any other nature does not exist. 
-Teleportation isn't a thing. (Why else would they take a spaceship if they could just teleport right over there?)

There are no magical means by which the crew can augment their existence on their journey.

What are realistic reasons for a spaceship to set out to settle on a new planet, that can only take eight (8) people on its journey? 

A:

They did not make their space ship.  They found it.
Your people do not have the tech to make ships capable of these speeds.  The 8 person ships they use for these voyages were discovered in an abandoned alien hangar, Gateway style.  The little ships were not built for beings the size of your protagonists and may not have been intended for manned voyages at all.  A lot of retrofitting is required to accommodate crew and needed supplies for these long voyages.
Scientists are trying hard to reverse engineer the near-luminal speed drives on these little ships, but so far without success.  Your people use the ships they have, not the ships they wish they had.  

A:

Keeping alive (without stasis) a certain number of people for 7 years, without access to any supplies, requires a lot of redundant systems and supplies. 
This means that you need a minimum amount of weight as life support hardware for person (in addiction to a fixed amount). I expect a formula like:
Total mass = (propulsion systems and miscellaneous) + (fixed weight of a life support system) + crew * (life support weight per person)
The problem is that the propulsion system doesn't scale well with the mass of the starship, so that after a given mass it is impossible to achieve enough thrust for the voyage*.
According to the calculations of the scientists, the maximum is 8 people, becuse more people would require a heavier ship, which in turn would require to oversize the engines too much. Even to the point that the higher thrust would require the engines to be able to bear forces beyond what current known materials can withstand.
-* I'm supposing that, even if a wizard can generate significant amounts of energy, the material of the engines can sustain and convert to thrust only a limited amount of energy

A:

The spaceship uses Direct Fusion Drives
A Direct Fusion Drive is a small fusion engine suitable for spacecraft, and it may be available as little as a decade from today. The drawback is that it can't be made very big - the limit is roughly 1 m width of the plasma chamber. A 10-megawatt fusion rocket engine will weigh about 10 metric tons and be 1.5 meters in diameter and 4 to 8 meters long. You can't make it much bigger than that.
You can add more engines, but there probably is a practical limit beyond which you get diminishing returns - and that limit may coincide with propelling a crew of 8 and the supplies they need to suffient velocities to travel the 2.3 light years in 7 years.
For just 8 people to start a colony (assuming they are human-like), the optimal would be to have 8 young women and a lot of frozen, fertilized eggs of high genetic variety. For the first maný generations, it will be optimal for population growth to have more females than males, especially as long as there are fertilised eggs. In fact, only the last batch of implanted eggs need produce males - and maybe the majority of these will have to be male to ensure enough variation in Y chromosomes in future generations.

