As far as I know, you don't have to submit MRs with your initial submission, because if you get in, these have to be resubmitted for consideration for the collection. It's not automatic that your submission images will be accepted for the collection.
Unless things have changed, you don't have to keyword your initial submission either. However, once accepted, you can get images rejected on keywords. I wouldn't get hung up on any specific number of keywords, it just depends on the image. I have a few with only two or three other than the generics, and a few I was pushed to keep under 50. Don't spam 'facts', and make sure any conceptual keywords are really relevant to your image. Don't list a colour because a tiny incidental thing is that colour.
If they (PLEASE) revive BM2, spam will get penalised, though there was a theory that sometimes good keywords also got knocked down by the system, depending on which got searched.

iStock has a controlled vocabulary, which is IMO generally a blessing (a view not always shared) and sometimes a curse.

Here are two iStock articles about keywording:

http://www.istockphoto.com/article_view.php?ID=227 Keywording manual

International keywording: http://www.istockphoto.com/article_view.php?ID=217