I don't think I could tell you specifically how many schools wear uniforms, however, I can tell you that there are more non-uniformed schools out there. Majority of the schools that require uniforms are catholic, but some are special program schools.
It depends on the type of school. Mostly public schools do not need uniforms. But in Catholic schools you almost always have to.AND IN MOST christian schools and private schools they about 90% of the time have to wear uniforms
In Spain its the same as here some do some don't although most of the school don't have school uniforms. The requirement to wear uniforms is much more common in Spanish private schools than in public schools.
The previous answer is very untrue. Public schools run by the state are not required to wear a uniform but in privately run schools most do wear uniforms or have a very strict dress code though some do not. It all depends on who it is run by.