Plasmid copy number (PCN), the average number of plasmids per bacterial cell, links gene dosage to key traits such as host fitness, virulence, antibiotic resistance and evolutionary potential. Although often viewed as static, PCN is a dynamic, regulated trait responsive to environmental cues and selection pressures. This review examines the regulatory mechanisms controlling PCN and its impact on the trade-offs between bacterial fitness, virulence cost, and antibiotic resistance.