The number of transfer students varies… Princeton takes none, while Cornell’s incoming transfer students number about a sixth of its number of incoming frosh.
Colleges vary in where they prefer transfer students to come from. While some super-selective schools may prefer to take them from other super-selective schools, Stanford historically has taken about half of its (few dozen per year) transfer students from community colleges (e.g. http://news.stanford.edu/news/2012/september/new-student-orientation-091712.html ).
High school record is more important for sophomore level transfers than for junior level transfers, since junior level transfers have more of a college record to show.