Is there an out of the box "reorder quantity" field? Not reorder point, but a field that would capture the suggested quantity to order upon hitting that threshold.

fixed lot size, but that’s only possible if you choose ‘Time Phased’ as a ‘Replenishment Method’. But then you loose the ‘Reorder Point’, although in that case you’ve the ‘Safety Stock Level’ which would be your ‘Reorder Point’. When you choose a ‘Time Phased Replenishment Method’, you’ll have to run the supply plan, which will propose you what to order.

The reorder multiple field works for reorder point items. This doesnt require demand planning