I am currently pursuing master degree in Germany and is applying to US PhDs for 2023Fall. I have heard it for long that for PhD applications letters from professors in class are not of much use. But my situation is that I have done several projects(with decent outcome) under only one professor, and I have no other professors to write letter for me in terms of research potential. Will I be in disadvantageous situation as many people have research experience in multiple groups and multiple letters talking about research?
For US applications I wouldn't worry too much about this. There are a lot of reasons why a professor with whom you'd only taken classes would be able to confidently predict your success. And several projects under your belt is a positive indicator in itself.
But, you are where you are and you use what you have. Don't ask for letters from those you aren't sure about, of course, but one very strong letter and a couple of others that are also strong but focused on more general things than research should be fine. Did you ask a lot of questions? Did you express a lot of interest, generally and/or specifically? Did you stand out in some (good) ways. Those sorts of things make you memorable and increase the likelihood of a helpful letter. Along with good marks, of course.
Note that admissions decisions are broad based in US, though letters are very important.