I know this is old but maybe this will help someone!
To see the answer to this question, which I asked last summer, go to Manual sync 5.802.
The option to download more than 25 is in settings. Select the email account and go to fetching mail/local folder size. If you only use K9 every few days set that folder size plenty big enough to fetch all the mail since the last time you fetched. Otherwise you may end up not fetching all the messages after the last time you had K9 open.
You go to other email accounts via the menu bars in the upper left.