The 20 Witnesses you refer to as 'active' are the top 20 who form the governance on the chain. There are many more active witnesses, you just need to look down the list to sew who is running the current version.
I have encountered a few people who think the chain ONLY has 20 'validators' aka witnesses and they thought that made for instability. It wasn't until I pointed out no, those are governance witnesses with dozens morning running witness servers that they got it.
Cheers @steevc and @shadowspub - I'm learning lots, and conversations like this are a great way for me to find out where the gaps are in my knowledge and to pick up information from people who know far more than I do. I really appreciate it !