How can I render a 9600x1080 composition from after effects

Did your client specify the video file format as well?

Are you working in five separate 1920x1080 Comps to be tiled from left to right?  Or, are you working in one 9600x1080 Comp and then separating that into five panels?

If the former, nest the five 1920x1080 Comps into a containing 9600x1080 Comp and distribute the five nested Comps from left to right.  Then, add this to the Render Queue (don't send it to Media Encoder).

If the latter, add the 9600x1080 Comp to the Render Queue.

Rendering at Best Settings/Lossless will yield a huge file, but it will be a self-contained file.  If on Mac, your resulting .mov file should come in at about 20MB per frame.  If on Windows, your resulting .avi should come in at about 40MB/frame.  When delivering high resolution renders like this, I usually include a low resolution files as well.  I'd probably go with a 1920x216 or 960x108 for the low res.

If your client would like to simply be able to see each panel playing side by side, consider nesting the 9600x1080 Comp into a 1920x1080 for preview purposes, scaling the nested Comp to the Comp width (Layer > Transform > Fit to containing Comp Width).  Then, add that to the Adobe Media Encoder Queue and encode at a H264 preset like Match Source - High Bitrate.

-Warren