No, you were right the first time. The subject is not “rioters;” it is “number.” So, “a number is,” or “a number is not.”
The subject of a sentence can never be found within a prepositional phrase such as “of the rioters.”
It doesn’t seem to spread very well outdoors. It’s 19 times less likely to spread between people outdoors vs indoors. There was a study that said out of something like 7,000 cases of transmission investigated, only 1 could be attributed to an outdoors encounter.
This is likely also why COVID-19 hasn’t been wiping out the homeless despite them typically not being in the best of health to begin with: they’re mostly outside and probably aren’t catching it.