The existing answers are pretty good, but there's a whole bunch of variables that haven't been mentioned yet. (As usual, I went full ham on this, so TL;DR at the bottom)
What is this community?
This is a big one. Your community could be colonists, sent to colonize some distant land/planet/whatever. If that's the case, they would most likely be 50 virile breeding pairs, hand-picked to produce the largest possible second generation, in order to increase the colony size as quickly as possible. If that's the case, you can assume a high starting growth rate, and a much higher end population.
But, they could also just be 100 people chosen completely at random. And if that's the case, then you will have to think about:
The demographics of your starting 100 people (let's call them Gen I) will have a profound effect on the number of children they have (Gen II), and that in turn will directly affect the population's growth rate for the remaining 1000 years. Change one male to a female, for example, and the ensuing butterfly effect could affect your final population by a significant, and possibly surprising margin.
@Masterzagh already noted that the ratio of men to women will affect the side of Gen II: the further you get from 50:50, the less potential breeding pairs there are, and the smaller Gen II will be. But there are a few other factors as well:
- Sexuality: I don't know the exact proportion, but chances are that at at least one member of Gen I will be homosexual or asexual. That obviously reduces your number of initial breeding pairs.
- Compatibility: Maybe you have a 50:50 male/female ratio, but who's to say that gives you 50 breeding pairs? Some of them may not be interested in any of their potential mates. Some of them might pair up but decide they don't want kids. Depending on how fast Gen I pairs up, you could have people who are left out because the people they liked were already taken and/or "friendzoned" them, or because they're not attracted to the ones who are left. Or...
- Age: What's the age spread of Gen I? If they're all, say, 18-30, you're golden. The wider the age spread, the more problems you're gonna have, not necessarily in terms of "X is menopausal, Y is 13, etc", but in terms of the age gaps between potential pairings. If this is bad enough, you're in trouble.
- Other Priorities: This might sound like a weird one, but bear with me. I'm assuming Gen I are starting a brand new colony in an area where previously, there was nothing. So at least some of them will be busy actually building that colony. You'll have people constructing buildings and infrastructure, people growing crops, people setting up systems of governance and drafting laws, people scouting the land, and so on. And some of these people may end up "married to the job", and simply not have the time to settle down (or the energy to enjoy doing so).
- Prior Relations: Depending on the circumstances of how Gen I came together to start a colony, it's entirely possible that some of them are related to one another. And that, of course, throws some of your potential breeding pairs right out of the window.
- Technology Level: A futuristic colony is going to have a much lower mortality rate than a medieval colony, because of advances in medicine, general safety, and so on. On the other hand, they also have access to contraception, meaning lower birth rates. So it's kind of a 50/50.
Combine all these factors together and you might even find yourself hitting what's called a "population bottleneck", where you don't have enough people for your colony to be self-sustaining without resorting to in-breeding. And as @fractalwrench already noted, having ninja'd me... that's very bad.
And all that is just the start. Once you've got your Gen II, assuming you haven't hit the bottleneck, you'll have to move on to Gen III, then Gen IV, and so on. And from there, as I mentioned earlier, your population will gradually spiral upwards and become very difficult to predict. It could go anywhere from the millions up to the billions. It's almost impossible to say. But it will be influenced greatly by that initial burst of population growth.
- Depending on who those 100 people are, your final population could be anywhere from 0 to in the billions.
- You will want to think really long and hard about who those 100 people are. Literally the fate of your entire 1000-year civilization depends on it.