I know the UCs only accept junior transfers (at least I think) but Boston College requires only 9 credits minimally to qualify as a transfer, which is less than one semester (that is, if we go with the assumption that there are ~15 credits in a semester).
What are some unis/colleges that are like BC in terms of transfer requirements?
You can apply for admission after one semester of college just about anywhere in the country if you want to. Whether you are considered a freshman applicant or a transfer applicant will depend on the place you are applying to. You need to check their policies. Most places do not require that wait until you have a certain number of college credits before applying for admission if you aren’t applying straight from HS.
I think many colleges allow you to apply after one semester. More weight will often be placed on your high school record with only one semester of college credit though, so your reasons for transfer may matter. For example, if it’s a question of fit, it might be easier than trying to transfer to a college you didn’t get a successful freshman admit to as your application (usually) wouldn’t be notably different from then.
If you apply to transfer during your first semester they will look at your high school stats: GPA, SAT/ACT because that is all that you have. If you were already denied as a freshman you will be denied as a transfer applicant too.