public access computing vs. OPACs

How does your library determine how many computers to “set aside” for OPAC-only use? Is that decision based on anything? At the library I used to work at, we had about 15 public access computers with fully five of them OPAC-only. The other ten computers were mobbed. TechnoBiblio looks at whose using which comptuers at San Francisco Public and has some questions as well.