This depends on a multitude of factors. An important one is mortality. 8 people settling a planet will face many dangers from wild animals over natural desasters to bad crops. How many children survive, and how long they will be able to reproduce is of course crucial to population growth.
Also, the people will, at least at first, have to spend all of their energy just to survive and to build up a stable settlement, they won't have a lot of effort to "waste" on raising children so this wouldn't be a priority for them.
Then: do you want to use your women as baby factories or should they actually be treated as humans?
Depending on all of these and other factors, you can get wildly varying numbers:
put your assumptions into http://www.wardricker.com/timegrowth.php to see for yourself:
If, as you have suggested, human beings have been on earth for 200
years and produced another generation each 20 years, that would mean
that 10 generations have passed since humans first "arrived" on earth.
If we started with 6 people, and each couple in each generation had
produced 10 children, with 50 percent of those living long enough to
in turn reproduce, the population of the earth for the current
generation would be 57,220 people.
-
If, as you have suggested, human beings have been on earth for 200
years and produced another generation each 15 years, that would mean
that 13 generations have passed since humans first "arrived" on earth.
If we started with 6 people, and each couple in each generation had
produced 20 children, with 100 percent of those living long enough to
in turn reproduce, the population of the earth for the current
generation would be 60,000,000,000,000 people.
Note: i plugged in 6 people, because the males are unimportant, so 6=3 couples.
All of this, of course, ignores the problem of the reduced gene pool here. I'm not sure how bad that would affect the population in just 200 years, but it sure wouldn't be pretty.