I don't have a good general picture of how much teaching is done (partly because I haven't studied in the US
A lot of teaching is done. It is easy to get the impression, when talking with faculty from the relatively small number of research institutions, that there is little teaching. But most institutions in the U.S. are not research institutions, and most institutions have teaching as their primary mission.
A two-course-per-semester load would be viewed as extremely light at most institutions; three to four courses per semester is common in mathematics departments at the university level. Higher loads are common at the community college level.
When applying for positions, one of the easiest faux pas to make is to try to negotiate an unrealistically low teaching load. It is unlikely that a school will give a new tenure-track professor a load significantly different than the existing ones have. But they will view the request as as a sign the applicant hasn't done their research, or a sign the applicant will not be a good fit.
So you want to know the typical load at a school as soon as possible in the interview process, preferably before you apply. When investigating the typical teaching load at a school, there are several important things to ask about:
Do they count courses, or do they count hours?
The "N + M" system of counting loads is not universal. Many schools count loads by the credit hour. In these schools, a 12-hour-per semester load is viewed as high, and not compatible with much research -- essentially a 4+4 load. But if the school has courses that are more than 3 credit hours, a 12-hour load might be a 3+3 load. In mathematics, the three calculus courses are often 4-hour (or even 5-hour) courses, and sometimes so is differential equations or linear algebra. You have to research this on a school-by-school basis. Schools with loads higher than 12 hours per semester are unlikely to require any research, but I have heard of some teaching-only schools with 15-hour (or higher) loads (e.g. community colleges).
To find out the "real" teaching load, look up the schedule of classes for the fall and spring and count how many courses the existing tenure track faculty are teaching.
Do they have "research releases" to reduce teaching loads?
Only the best research schools will guarantee a light teaching load. Many schools have a uniform, heavy load for everyone - but then give "research release time" to reduce the teaching load of faculty who are active in research.
This was true even at the highly ranked institution where I did my PhD - the default load was used mainly for older faculty who were no longer active in research, while the lighter load was used for research active faculty. Research releases have become common even at non-research-intensive schools that want to increase their research profile. In almost all cases, they are not written into the contract, and are handled by a separate policy. So you also have to investigate these on a school-by-school basis.
For example, at my institution the default load is 12 hours per semester, which is typical for this type of institution. But research active faculty receive a reduction to 8-9 hours per semester. Several freshman and sophomore level classes, including precalculus, calculus, and others are 4-hour or 5-hour classes. So, depending on the way the course schedules turn out, the 12-hour load is often a 3+3 load, and the 8-9 hour load can be a 2-2, 2-3, or 3-3 load. For me, this is a quite reasonable load which allows me to publish regularly as well as teach several courses.
Some schools that otherwise have a high teaching load give a research release for first-year faculty, to give them a chance to write up the results of their dissertation and publish those.
How often do you get a sabbatical?
A sabbatical is a semester or year of modified duties, typically with no teaching.
Some institutions guarantee a sabbatical before tenure, which can help offset a higher teaching load. The main examples I've seen of this are small liberal arts colleges. At other institutions, you will never get a sabbatical before tenure (e.g. regional public universities). So you need to investigate the sabbatical policy, as well, because a semester or year of no teaching significantly affects your average teaching load in the surrounding years.
Is there a good balance?
The final question about teaching load is whether there is a balance between the amount of teaching and research you would like to do, the amount of teaching and research you are expected to do, and the amount of teaching and research you are able to do with the teaching load at the institution. Low teaching loads, on their own, are not a guarantee of happiness!
