If the sponsor relies on a salary and has been with their current employer for at least six months, six months of personal bank statements matching six months of payslips. If they've been with the employer for less than six months, the evidence also has to show the income over the previous 12 months.
Appendix FM-SE asks for personal bank statements covering the same period as the payslips, so the salary can be seen arriving each month. The minimum income for new applicants is £29,000 a year. Six monthly statements is the normal minimum for a salaried sponsor, and if the statements are in French, Spanish or Urdu, that's six statements to translate. Our payslip translation guide explains how the payslips, employer letter and statements fit together.
The twelve-month version is for sponsors who changed jobs recently. They show the current job and the earnings over the previous year, which can double the paperwork. Self-employed sponsors have a different set of rules again, built around full financial years and tax evidence, so check the guidance for that route before counting months.