Using "twitter.retweet.count > n" will result in the retweet being returned each time that same Tweet is retweeted, so you will receive an interaction when it is retweeted n+1 times, n+2, n+3..... It might be more sensible to use the following CSDL:
interaction.content contains "keyword" and twitter.retweet.count in [10, 50, 100]
This means you will receive an interaction when a Tweet matching your keyword search is retweeted 10, 50 and 100 times - you will not receive every retweet.
The original Tweet that was retweeted can be found in the twitter.retweeted.* part of the interaction. Take a look at this sample retweet interaction for an example. In this case, @RWW sent the original Tweet, and @nik retweeted it.