The nurse-to-patient ratio planning presents a global challenge in healthcare systems, with significant disparities across countries. It is a widely studied phenomenon, yet methodological and conceptual gaps persist. In Spain, the nurse-to-1000-inhabitant ratio (6.2) remains below the OECD average (8.8), resulting in an estimated shortage of 100,000 nurses. This issue is exacerbated by political decisions influenced by economic, cultural, and organisational factors. This work aims to review existing methodological approaches for determining nurse staffing levels, identifying their strengths, limitations, and potential improvements to ensure the efficient and safe allocation of resources. Five methodological approaches are analysed: expert judgement, distribution-based methods, time measurement, correlation between staffing levels and adverse events, and stratification by patient complexity. National and international data are compared, and their impact on safety, efficiency, and costs is assessed. The study confirms that higher nurse staffing levels reduce mortality and adverse events. Models based on patient complexity, such as INICIARE, provide a more precise and adaptable approach. In conclusion, nurse staffing planning should be based on a model that stratifies patient complexity levels according to care dependency while minimising institutional variability. It should be linked to clinical outcomes, patient safety, staff competencies, and workforce stability. Additionally, research should extend to primary and social care settings, where evidence remains limited.