I am planning to apply for ISO 9001 certification. What are its requirements in terms of measuring instruments?

The requirements are specified in paragraph 7.6 of the standard, ‘Control of monitoring and measurement equipment’. These requirements are similar to those found in the standard ISO/IEC 17025 as applicable to laboratories, i.e. determining the measurements and measuring instruments that impact (or can impact) the validity of the measurement, test or inspection results.