Sorry, what you wrote makes no sense.
Are you asking about how to use Rhythm Pattern?
Are you asking about 8 Arps?
Your Motif XS could not save eight Arps — you are mistaken.

Please try again. Use the following example, so you know how it should work...

Recall “CFX Concert
This is a four Part Acoustic Piano Performance — let’s add a Rhythm Pattern to this piano...
On the HOME screen - you can see the four Part slots active (KBD CTRL icons are green)
Press [RHYTHM PATTERN]
This will add a Drum Kit to “Part 5” and launch the Rhythm Pattern routine

It places you on a screen called “Part 5 Category Search” for Drum Kits by name.
Select a Drum Kit — it will assign the Arp Phrases that are stored with the Kit you select to Arp Select 1-8 (the actual number of Arp Phrases stored may vary. You are free to associate your own selections in addition to or in place of those currently assigned.
Select a Rock Kit for Rock Arp Phrases. Select a Jazz Kit for Jazz Phrases, and so on, etc., etc.

If your Performance already has filled the eight KBD CTRL Parts you cannot ‘add’ a Drum Kit (via [RHYTHM PATTERN] routine).

Drum Kits can be manually added to a Performance
And Arp Phrases can manually be assigned to any Part (drum or non drum).
Pressing the [RHYTHM PATTERN] button is the same as tapping the “+” to “merge” a new Performance with the “CFX Concert” Performance. Had you tapped the “+” in the empty Part 5, and then selected “Drum/Perc” as the Search Category, you would arrive in the same place... but in addition the Part Arp Switch is set to “Arp Hold”, and KBD CTRL has automatically been activated... Many of the Drum Kits already have Arp Phrases associated with them — associated means temporary - you can, of course, choose any other Arp Phrase. These are all things you would have to do each time you wished to ADD (merge) a Drum Kit to a factory Performance that didn’t have one already.

A maximum of 8 Parts can be under KBD CTRL at a time
Drum Kits and Arpeggios can be assigned to any Part 1 - 16 (a Drum Kit assigned to a Part 9-16 will need to be *selected* in order so communicate with it... all Parts 9 thru 16 function as single Part instruments.