Is neurofibromatosis a rare disease?

Define rare. At 1/4000 births this is relatively common compared to many genetically influenced afflictions. About a 3rd are new mutations and the others are passed from an affected parent to half their offspring.
Not so rare. Estimated incidence of NF1 is 1 in 3000, but the actual frequency may be higher because of less than complete identification of mildly affected individuals. Approximately half of affected individuals represent first cases in the family as a result of a new genetic event or mutation. NF1 gene located on chromosome 17q11.2.