The physical size of cylinder/s (not 'bottle/s') will be dictated by the volume of nitrogen required to fully inflate the float bags. If a single cylinder of this size can be installed in the airframe at a convenient location without unduly affecting the helicopters centre of gravity then this will be the simplest solution. However, if a single cylinder cannot be installed - for whatever reason, including the necessary plumbing - then the manufacturer must look at multiple cylinders.
For example, early JetRangers had a dual cylinder arrangement in the baggage compartment. Later versions, including the LongRanger series, had a single cylinder under the fuselage - even this had two arrangements, some helicopters had it installed along the longitudinal axis while others had it installed along the lateral axis. This seems to have come full circle now as the latest 407's have two cylinders mounted in parallel under the fuselage along the longitudinal axis
As for "What percentage of light/small helicopters (R-44 or R-66) is equipped with inflatable emergency flotation systems?" the short simple answer is 'who knows', perhaps an e-mail to the factory will help.