How many swear words should you have in a conlang, and how are they designed? What is their purpose really? And should any other "regular" words be constructed out of them? Like if "bar" was a swear word, then "foobar" was a goose or "barfoo" was a house, is that a problem? Also, what are swear words actually, do they have a consistent meaning across cultures?
As jlawler mentioned in a comment, every culture has "taboos" (also sometimes spelled "tabu", borrowed from Polynesian). The term usually refers to objects and actions that are somehow culturally forbidden or repulsive, but it can also refer to concepts and words.
What words are taboo varies a lot by culture. Nowadays in American English they tend to refer to sex and bodily functions (fuck, shit, ass, slut). In other, usually older varieties of English, they had more to do with religion (damn, hell, bloody). In Classical Latin you would swear by a god or a demigod (one of the most common expletives is usually translated "by Hercules"). In some long-ago ancestor of English, the Proto-Indo-European word for "bear" seems to have become taboo, which is why we say "bear" (possibly from a root for "brown", possibly from a root for "wild animal", there's not a consensus) instead of some cognate with Greek arktos.
The one constant, though, is that there'll always be some sort of taboo (both culturally and linguistically). If you're inventing a conculture, decide what they consider forbidden. If you're tying your conlang to an existing culture, look at what words they swear by. But in a natural language, there will always be something that's considered offensive.