Depends on the subject. But yeah, we run a Just In Time process for this
Basically, this means that the Uni is pretty slow telling us how many students we'll have, so we don't know numbers till about now. e.g. on Friday, AFM1A was 176. Today, it was 206. You never now.
However, we do allocate class preferences to full time staff first, usually. But we still won't tell you because students are often "deluded" into believing (wonder why...) that some staff are worse than others. Seriously, years ago, there was once a bunch of students who would suddenly migrate every time the lecturer changed during session, coz they wanted to be in the current lecturer's tute (and we usually have four lecturers in main session 1A).
So, the short answer is: we can't tell you, but if even if we could, we wouldn't
Don't know if that applies to other schools.
Cheers,
RobertC