1) If you use several social media channels, it is likely that there will be some overlap in the reach achieved by each channel, which will result in double counting the people you reached. For example, among 200,000 people reached on one social media channel, many will also be reached by other channels. Unless your campaign can quantify the overlap (e.g. by surveying the target audience members), the best thing you can do is to 1) acknowledge that there might be an overlap; and 2) report on the reach achieved by the individual channels in addition to reporting the total reach.
2) Some posts might have a higher reach than others. Does it mean that you should use more of them? Not necessarily – people might give them more attention because of how they’re designed or because they are funny, but they might not do well in getting people to click on your links or follow your content (which is what you are likely to be interested in the most). Always use this indicator in conjunction with indicators that measure active engagement, such as “shares”, “clicks” and “comments”.