One syn per 20 calories, and the calorie value should be on the packaging.
According to the Ocado website, the 35g packs which have snack pack written on the outside and come in a pack of four, are 165 calories per 35g bag, which would equate to eight and a quarter syns.
There are also mini fingers in 22g packs which are 115 calories, which equates to five and three quarter syns.