Having multiple clients is often seen as a sign of entrepreneurship. However, the Tax Authority (Belastingdienst) has repeatedly stated that it is not problematic to work for just one large client, even if that means having only one client in a given year.
It is not necessary to have 2, 3, or 4 clients. Ultimately, it depends on the type of assignment you perform and how you behave as an entrepreneur in the economic market, i.e., meeting the entrepreneurial criteria. Of course, the specific circumstances may vary from case to case, so it’s always important to stay well-informed and review the criteria more than once a year.