The 'one per exit' rule seems to be some old Transport Command thing...
When studying Air Law for my R/BCPL/FI so that I could provide remunerated flight instruction on Cherokees, I learned that the civil world requires one CC per 50 pax or fraction thereof, whenever one or more passengers are carried. One of the more irrelevant parts of the Air Law syllabus for PPL instructors...
So in a CAT aeroplane with 149 seats and half a dozen passengers, the same 3 CC are required as for 149 passengers. There will probably be 8 emergency exits on such aircraft, so if you're travelling in an overwing window seat in an emergency exit row, you should be given a brief from the CC about how to operate the overwing exit. The last time I flew in such circumstances with Lufthansa, I was given a very professional brief from a most efficient young lady CC, including the specific PA announcement which would precede evacuation.
8 CC in a Voyager with 106 passengers does seem to be rather OTT?
Lufthansa used to carry more CC than JAR-OPS required, but that was back in the halcyon pre-Apr 2004 days before the LoCos dumbed airline travel down to its current dire level, so that those of us lucky enough to be travelling in biz class could enjoy a high quality service. Birmingham to Frankfurt meant a large gin and tonic and some almonds shortly after the belt signs were extinguished, followed by a 3-course hot meal with as much wine as one wished, then coffee and brandy plus a couple of choccie truffles. Those were the days!