Is there a limit to the number of cyclones that can be spawned from Sweeping Wind?
I don't believe there is a limit. If there is one, it's quite likely to be unreachable.
Here is a screenshot of 5 simultaneous cyclones made using a 21% crit chance and several crits per second.
screenshot http://cloud.steampowered.com/ugc/558692204679386275/A0715CB169CE0AA81FAA7A28F431A6600DB23151/
You have to remember that not only do they only spawn when you crit, but it's also a chance to spawn. Getting many more than 5 is very unlikely, they despawn very quickly.