Much depends on rider weight, rim width, tire size and shape and the quality and tension of the build. The Moulton F frame and Mark III original standard on steel 349 rims is 28 hole. These have proven very durable for me on my 1965 Moulton Stowaway and 1974 Mark III.
I am a large person so you may be able to manage with fewer spokes if you are not a clydesdale. I have no experience to offer on reduced spoke numbers on 349 wheels.