Why is the cheque number zero when using a cheque as a payment means?

baobao

By definition, the cheque number is updated only after a cheque has been printed & then confirmed.

When using pre-numbered cheques functionality, the first cheque number is defined in the house bank setup. All cheques printed for that particular bank account will then follow in increasing sequence.
In this example, the cheque is used to pay an AP Invoice over GBP 15.00. The house bank account shows that the next cheque number should be '8':

After adding an outgoing payment, the cheque may be displayed by selecting Banking -> Outgoing Payments -> Cheques for Payment:

The cheque number (Cheque No.) is still '0' since it is not confirmed yet that this cheque will actually be printed onto the pre-numbered cheque. At this stage the cheque may be voided or cancelled without affecting the cheque numbering.
When the 'Print' command is given, SAP Business One will display a system message, alerting the user to ascertain that the correct paper with the correct cheque number is present in the printer:

Once the cheque has been printed, the user is prompted to confirm the actual cheque number in this window:

At this stage the cheque number can be changed:

Once the cheque number has been checked & verified, the print status is changed to 'Confirmed'.
Only now the cheque has a permanently assigned number: