bank wants to know how many staff my business has why is that necessary

Number of employees determines the CSRD size category

The bank usually does not ask this out of curiosity, but to determine which category your undertaking falls into for sustainability reporting. The number of employees is, together with turnover and total assets, one of the thresholds in the CSRD (Directive (EU) 2022/2464). Banks must gain insight into the chain through their own reporting obligations, and the size of a supplier determines what level of detail they are allowed to request. A microenterprise logically receives a lighter questionnaire than a medium-sized supplier.

In practice, this works through in the questions themselves: many bank portals use the VSME basic module from EFRAG as a template, and that module asks for the number of FTEs as the first indicator to scale the reporting burden. If you do not have this figure readily available, the Chamber of Commerce is usually the fastest source: those numbers are fixed on the basis of the Trade Register Act 2007 and are traceable for the bank. Note that "number of employees" is not defined the same everywhere — sometimes FTE is requested, sometimes headcount at the end of the financial year. That distinction is worth checking before you fill in a figure, as it often makes the difference between being above or below a threshold. This is incidentally related to the company's annual turnoveras both figures are usually combined in the same question to determine the size category.

Where this follows from the VSME module and the directive

The VSME basic module from EFRAG names the number of employees (expressed in FTE) as the first size indicator, alongside turnover and total assets, to determine which level of reporting applies. The CSRD (Directive (EU) 2022/2464) uses the same three criteria to distinguish between micro, small, medium and large undertakings, and banks translate those thresholds into their own chain questionnaires. The underlying figure itself is usually traceable from the entry in the trade register under the Trade Register Act 2007. For completeness: this number of employees is often requested in the questionnaire alongside the company's registration number because the bank needs both pieces of information to identify and categorise the undertaking unambiguously.