Fill out the Psychology Major Worksheet (downloadable here) and email or bring it to the DUS. We also STRONGLY recommend that you take statistics (Psych 200) and a research methods course (PSYC 201-299) BEFORE you study abroad because you need to take these courses before you start your senior year if you are doing a BS degree. Also, these courses are rarely offered in other foreign institutions.
We don't have a limit. But see the next question.
In consultation with the DUS, you can select courses that have a substantial amount of Psychology content. In order to receive official approval from the DUS to count those courses toward your psychology credits, you will need to submit the syllabi, which include reading lists and requirements. In most cases, however, those materials are not available when you apply for study abroad programs.