Most students are advised to apply to at least 8-10 graduate programs. As when applying to undergraduate schools, this should include a mix of “safe” schools, good matches, and “stretch” schools.
Note that these choices depend on the academic reputation of the graduate programs you are applying to, and not the institution as a whole. Several web sites provide rankings of graduate programs in computing, and even in specific research areas. Rankings, however, are not a good indicator of what your experience will be: if you’re applying for a PhD program, that will be greatly determined by who your advisor is, and often not as much by the institution that you’re at.
Students should talk to faculty and advisers to get additional feedback and suggestions of schools that are good matches with their interests and abilities.