To change it, go to the Settings page -> Data tab. When data is segmented by visit history (number of visits), charts can become difficult to read when there are more than 20 categories. This setting allows you to apply an upper bucket to group visit counts of that specified number or more. By default this value is set to 20 such that you will see "20+" on charts with absolute and relative visit history segmenting. That bucket will contain visit counts of 20, 21, 22, … up to the largest visit count in the requested data. If you prefer not to have a maximum bucket, please activate the "No Max Bucket" toggle.