Fairly often. There are a couple ways that supernovae get spotted. Sometimes an amateur astronomer will find a supernova, there are lots of folks who actually do this as a hobby and surprisingly it's something that such folks can actually be fairly productive about, but it requires a lot of work. Also, there are various ongoing sky surveys of various sorts including asteroid detection programs which typically pick up supernova at a fairly regular rate merely due to being able to observe a huge number of galaxies in any given night (since supernovae are typically bright for periods of many days to weeks). Additionally, we've deployed a variety of satellites that have "transient detectors" designed to look for the signs of gamma ray bursts but also capable of detecting ordinary supernovae, sometimes.

For example, in 2013 over 230 supernovae were discovered. And currently there are about 3 dozen active supernovae that can be viewed with a telescope, but most of them are fairly dim in very distant galaxies and would require a fairly sizable telescope to be able to see (~300mm aperture or so). The M82 supernova is the only one that is currently visible with an ordinary backyard telescope or binoculars.