how do I estimate the expected transaction volume for the bank

Expected transaction volume is an estimate, not a financial statement

Enter a realistic estimate of the amount that flows monthly through the business account with this customer, broken down into incoming and outgoing money. The bank does not use this figure to verify whether it is correct down to the euro, but to establish a transaction profile: does the money flow that will come in match the picture the enterprise has sketched in advance. A rough estimate based on last year's turnover, divided by twelve and adjusted for seasonal patterns, is sufficient. There is no legal definition of "correct" here — it is an expectation, not a determination.

If you have no history, for example with a new account or a newly started business, then a substantiated estimate based on the sales forecast is the best alternative. Stay reasonable: a volume that deviates significantly from the business size that appears from other answers — the filed financial statement for example, or the description of business activities in the business register — is more likely to attract a question from the bank than a cautious estimate that you can adjust upward later. This figure is also separate from the question of the source of funds": that is about where the money originally came from, this is about the ongoing flow.

Where this client due diligence originates from

This question does not come from a bank form in itself, but from the obligation to conduct client due diligence under the Anti-Money Laundering and Terrorist Financing Act. That law requires banks to record the expected use of an account, so that deviant use can be detected later. The transaction volume is thus part of the risk profile that the bank builds of a business customer, not an accounting figure that is verified against a source anywhere. There is therefore no source document that "confirms" this figure — unlike a question about UBOs, where the business register or the chain file can provide a recorded answer.