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