There is no minimum or maximum number of patients stipulated, however your sample should include current/recent patients. The purpose of audit is improved patient care through application of recognised standards and ongoing efforts to sustain this change. If you can show that you have considered the clinical management of patients with reference to an agreed standard, you have already conducted an audit. However the current recommendations of the Medical Council are that you should spend one hour per month on audit activity.
If you can extract data easily from your practice management software system for your audit, it is feasible that you could conduct your audit on all relevant patients (e.g. all with COPD or all with Diabetes etc.), however, if not, you can conduct your audit on a sample of relevant patients. There is no specific guideline on the numbers, however, since you will be basing decisions and changes on the findings of the audit, you should ensure that the sample is of sufficient size and selected in an appropriate manner so that the findings are valid and reliable.
Source: ICGP Clinical Audit Toolkit