Was looking for the same thing and found a way to do this, thanks to the forum software docs
. Replying to this post here so that it helps others too.
1) Subscribe to more than default 5 posts:
Add ";limit=n" to the URL.
2) Subscribe to multiple boards with a single feed:
Add ";boards=x,y,z;limit=n" to the URL.
x,y,z are the board IDs. These can be got from the board URLs on the index page
Note that it should be "boards
", not "board".
I'll probably write a python script to fetch RSS feeds to all boards and generate a OPML file which can be imported into feedreader. Will keep posted on any progress.