You should definitely try to have a minimum of 5 syns a day but if it is a one off then I wouldn't worry too much. They are there to ensure you don't feel deprived. It's that old adage of 'a little of something you fancy does you good'. If you don't have those things you enjoy - whether that be chocolate, alcohol, cheese, bread, sauces, eating out or whatever - then the likelihood is that you'll eventually get fed up and fall off the wagon big time. Also some people find that as they near their target they have to cut back a bit on their syns and if you're not having any or very few then you have nowhere to go. So I would aim for 15 and if you continue to lose weight at an acceptable rate then why not enjoy them.