FWIW, Ascend database shows 280 Saab 340s (all models) in service and a further 119 in storage. That's a total of 399 which is probably SAL's "marketing" number.
Bear in mind that the potential replacement market for a (hypothetical) new-build turboprop in this size category is not just existing SF340 operators but also many existing operators of Dash 8-100/200s (325 in service), E120s (197 in service), Do328s (121 in service), J41s (70 in service),...