Since about 1990 astronomers been able to detect planets around other stars, using a couple different techniques, which is amazing.
By this point, do we know whether planets form only around single systems, only around multiple systems, or both??
(Surprisingly you can't find this general issue explained in popular writing. I think it's "obvious background knowledge" to experts, hence just doesn't get outlined explicitly in popular writing.)
So, as of mid 2016:
(i) About how many exoplanets do we know of (say, with good certainty),
(ii) In fact, how many (if any) of these are around single-star systems?
(iii) How many (if any) are around multiple-star systems?
If indeed it is possible to have planets in multiple systems: is it just a planet going around one of the stars, or is "all the way around both" known?