There are no specified limits for public assemblies in US.
The government expects peaceful assembly among others and the government can impose special restrictions if required which includes time or location or number of people.
Explanation:
Assembly is covered by the freedom of speech in the constitution (first amendment). This means any group of people gathered either for political or entertainment reasons can do so freely. However, law enforcement has the right to disperse any assembly that is suspected to lack peace or may lead to riots and other forms of violence.
Another restriction is the assembly that may pose harm to the general public. Protestors who cause nuisance do get arrested by the police. Assembly, as enforced by the courts, must be legitimate occurrence and should address important topics like public safety.
