insurer asks who my main customers are must I disclose that

Customer concentration is a risk assessment question, not a secret

Yes, in practice this is usually shared, but mostly in a form that does not contain names. An insurer asking this — for example with trade credit insurance, commercial property insurance or a liability policy — wants to know how dependent your business is on a small number of customers. This is called customer concentration risk: if one customer represents 60% of your turnover and that customer leaves or does not pay, that is more relevant to the insurer than with a customer with a hundred small customers. So the question is rarely about who your customers are, but about how concentrated your turnover is.

In practice you can often answer this with a percentage or a range — "largest customer is less than 20% of turnover" — without naming names. Some portals explicitly ask for the identity of the largest customers, for example with trade credit insurance where the insurer assesses debtor risk per customer. Then a name is part of the question. If you are unsure whether a name is necessary or whether a percentage is sufficient, that is a question for the customer who issues the list, not something we make a statement about. Note that this is different from the company's annual turnover providing: turnover is a total figure, customer concentration is a distribution.

Where this comes from: no legal obligation, but market practice

There is no law that requires an insurer to ask about customer concentration, and there is no law that requires you to share names — this goes through the policy conditions and the insurer's own acceptance procedure. What is fixed are the basic data of your business, as registered under the Trade Register Act 2007: name, registration number and sector. Customer concentration is not listed there — that is additional information that the insurer requests separately for risk assessment. If you are unsure what a specific portal exactly means by "main customers", the question formulation of that insurer itself is the first place to look.