I remember in my Boston, Mass. days (1976-79) going over to Brattle Square in Cambridge (a block down from Harvard Square) on a Saturday night in the summer and taking in more than a dozen different busking groups in an evening. It got to be such a popular place that I understand the city of Cambridge had to start a licensing system combined with a lottery to allocate the most popular sites. (The city, I should mention, invested some money in creating plazas and "street furniture" conducive to busking.)

I've never seen elsewhere such a concentrated collection of buskers.