They run 4, 5, 8 & 9 car voyager formations regularly. However, nothing is booked for 10 cars as there are platform length issues at several locations. That's not to say it never happens, but they always try to avoid it if at all possible.
The current influx of double units is a result of covid. The South West services have been reduced freeing up more units thus meaning the Bournemouths can (mostly) be doubled. Should the South West services resume, then single 4 & 5 coach voyagers will once again be the norm.