what must I enter for expected transaction volume for the bank

Expected transaction volume is an estimate, not an annual figure

You enter expected transaction volume as a reasonable estimate, substantiated by what is already known, not as an exact amount that must be precise to the euro afterwards. The bank asks for this in the context of customer due diligence under the Act on the prevention of money laundering and financing of terrorism: they must know whether the payments that will subsequently go through the account match the business you are. A discrepancy between estimate and reality is not a problem in itself; an estimate that does not align with your turnover or sector is.

As a basis you use the turnover from the latest annual accounts, supplemented by what you know about growth or new customers this year. If you have already filed that annual accounts with the trade register, then you can refer to it directly as substantiation. If an official figure is missing, for example in a newly started business, you provide a substantiated estimate based on orders, quotations or a forecast, and state that it is an estimate. Some banks ask for the amount per month, others per year; note which period the portal of your client asks for, as that changes the figure considerably without the question being materially different. This question often appears in the same section as the question about the origin of the company's assets: banks review both answers together to see if the cash flow picture makes sense.

Where this follows from: customer due diligence under the Wwft

The bank's obligation to ask this stems from customer due diligence in the AMLA: they must draw up a risk profile of the business relationship, and the expected use of the account — including transaction volume — is part of this. The underlying business data, such as the annual accounts on which you base the estimate, are established under the Trade Register Act 2007.