Well it depends on what uni you want to read law at. For Oxbridge, they may frown upon the odd C, as most applicants have straight A's and A*s, but having said that, they are far more interested in your AS grades, prediction, interviews etc. GCSEs carry a relatively small weight. It may also depend what the C's are in; it may be more of an issue if the C was in English than if it were in Tech. As long as you meet the general matriculation requirements of the uni, and think you can do well at AS and A2, then you shouldn't be put off applying anywhere,