From the Help Text searching for "default quantity":
Min/max inventory levels - IC Items only
Based on your Update stock levels on entry setting on the System Options tab of the Company Profile, you may see reorder suggestions for the selected IC item display in bold to the right of the Unit Price field. These suggestions (eg. Suggested qty: 5 or Overstocked) are based on the IC Item selected, the Location selected, the Quantities information displayed in the grid for the item and the Update stock levels on entry option setting in the Company Profile.
For example,if the option is set to ToMin or ToMax, you will also find that Adagio PurchaseOrders will automatically enter the suggested quantity to bring stock levels up to the Reorder Minimum or Reorder Maximum quantity for that item at the selected Location. As this is a suggestion only, you may choose to enter a different quantity to order.
Set the option in the company profile so that a default reorder quantity of 1 will be suggested. Use "To minimum" and set the minimum level to 1 for the item (on the assumption that you are raising a PO to get an item for a customer.