You can not import a number of posts from feeds which exceed the number of time slots assigned. If you attempt to, an alert will load. Add more time slots and then increase the number of posts to import from your RSS Feed.
SETTING HOW MANY ITEMS (POSTS) TO IMPORT PER DAY
Hit on the "Feeds" filter and select as little as one item (post) and as many as 5 items (posts) to import per day. This setting only relates to the RSS Feed to the left of the filter and does not affect your other RSS Feeds.