This has changed in v19 and it is set for each vendor:

1. On Order: Replenishment needs will be grouped together (except for MTO).
2. Daily: Replenishment needs will be grouped if the expected arrival is the same day
3. Weekly: Replenishment needs will be grouped if the expected arrival is the same week or weekday
4. Always: Replenishment needs will always be grouped (including for MTO)

The difference between the first and last options is that “Always” will merge demand even if MTO is selected, whereas “On Order” creates separate RFQs for MTO.

Options 2 and 3 are new, and could useful if you set up Lead Times.

• One of the common complaints was that you might not want to send your supplier a Purchase Order for goods that were expected to be delivered to very different timetables.
• Of course, Odoo can handle this with backorders, but it might be confusing to both the supplier / vendor and to internal users.

As far as I know, in v18 this is not possible.  In earlier versions you can add a procurement group to the reordering rule