To select the quantity when you add a product to the sale, turn on the quantity button at the bottom of the left side of your sale interface.
You can activate by default this button in 'Settings/General settings/Misc'.

If you scan barcodes or if you type your product codes in the 'Add a product' field of the sale, you can also use the multiplication sign *
Ex: to add 10 times product 1234, type 10*1234.
Ex2: for a barcode, type 10* and scan the barcode.

If you search for your products in the 'Add a product' field you can enter the quantity before your search.
ex: 3*shirt and then select the shirt among results.

To modify the quantity of a product in a sale, select the product line and update the 'Quantity' field.

For decimal quantities (weight, meters…) you must manage your product in the smallest quantity of sale and add integer quantities. More information.