I forgot how many solitary bees/wasps are in the US but it is close to 1000 I think. They do nest communally because of favorable locations, soil, and finding mates.
It is estimated that there are between 1500 and 1800 known and something like up to 2500 individual types solitary bees in the U.S. alone. Some are specific to a particular valley or mountain range.
I took part several years ago with a solitary bee study, that continues at PennState to this day. I think they add several new varieties of bees every year. Not sure who is heading up the study.
As with all of nature, unless health or safety is an issue, let them alone.