There is no hard and fast rule - you just have to find what suits you. There is plenty of anecdotal evidence on here that people have better losses when they have all their Syns, but there are plenty of people who don't, and I'm one of them!
If you're happy with 10-12 a day, then that's great and you shouldn't use more just for the sake of using them. However, if you use all 10-12 on what I call treat items (crisps/chocolate/wine, etc), consider using some on the little things that make a meal nice eg gravy, small Yorkshire pudding, salad dressing etc. as this is good for an interesting diet and helps with maintenance later on.
Do what you feel is right for you and, most important, what makes SW sustainable for you.