By default, not more than 250 cards are shown in a planning board swimlane. However, as a site administrator, you can increase or decrease the number of cards shown in the planning board swimlanes by configuring the site options token, PLANNING_BOARD_SWIM_LANE_LIMIT.
A positive number.
250
When you select a planning folder in one of the swimlanes and if X is greater than N, (where X = number of artifacts in the selected planning folder and N = PLANNING_BOARD_SWIM_LANE_LIMIT), the message, Swimlanes in the Board View is currently configured to show N artifacts only, appears at the bottom of the swimlane.