There is no easy answer. Experience will give you the answer and it will vary for every location.
Probably not the answer you wanted.
On that stretch of stream you might find that perfect set or two for beaver and that is all you will put in. On the other hand you might have a hard time finding what you think of as a great set so end up putting in a few more sets. And then there is the time factor. Maybe it is close by and you are happy to trap it over a long time, then maybe one trap. Could be you want to set it and be done in one or two checks, then you need to drop in more sets. Want to catch some otter there? Then you are going to stick around longer and be more selective with your sets.
Time of year is a factor too. Are they breeding and moving more?
See why it's a hard question to answer?
On rats though I would set every good run and move after a couple checks but if they are running you might find a few spots that will keep getting traveling rats. Again experience!