Twitter – Tweet responses

The answers so far will only give you at most 100 retweets. There is an API method that returns up to 100 entries of a cursored collection (so you can page through it to get more), though.

There's more information on the Twitter Developers site at GET statuses/retweeters/ids, though you'll also want to read their page about Using cursors to navigate collections.

Edited to add: That said, the first thread on the Developers site that surfaced in a quick google has @episod, a Twitter employee saying:

You can't likely get to all of them. The data is spread out. Your best bet is to use the REST API to determine many of the users who performed the retweet, but the data will still be constrained.

It's easiest to track retweets as they happen rather than try to find them from the past. Use the Streaming API for that.

^{Source: how to get all retweet of a given tweet}

Well I think the tweets that are listed (in case of the ones of celebrities) doesn't have any particular order.

Taking this tweet as an example I think notice that most tweets that are listed either are:

1. Having a huge conversation related with them i.e many other users reply to that tweet and it's a talk(which may be off topic).
2. Have been retweeted (not always the case).
3. User who tweeted has many followers.

These were the general trends I observed in this tweet. The listing by Twitter in most cases might be in fact completely random (no concrete way of knowing).