How many students you take will depend on how big your facility is, I'd say anywhere from 1 - 5 is average depending on size. Rotation length will depend on the particular school, usually from 5 - 8 weeks. Getting an accredited residency is a completely separate issue from students....generally most places big enough to have a resident will also have students.
I've never known a preceptor to spend 100% of their time with a student. A preceptor may have a student shadow them a day or 2, and the preceptor will check in regularly with the student, but the majority of the rotation will be spent having the student work with other pharmacists or technicians in the various areas of the hospital, or having the student work on a project independently. Taking vacation time shouldn't be an issue, just make sure the student know what they should do in your absence (and share that schedule with another full-time pharmacist, so if there are any issues, that pharmacist will be able to see what the student should be doing.)