What is the total number of nodes in the global bitcoin network?
Unknown, and impossible to know.
It specifically states:
"Bitnodes is currently being developed to
estimate the size of the Bitcoin network by finding all the
reachable nodes in the network"
After searching through their node list, I can confirm that my node is not listed there, so clearly it doesn't know the exact number. I have no way of knowing how accurate their "estimate" is.
I thought there were more than 30K nodes earlier this year,
You might be right. How would we know?
but the above website says 8K-9K
They might be right. How would we know?
Is there any other website with historical info?
blockchain.info might have some historical info about nodes that they have connected to. I'm not sure if there are any other services providing historical info about nodes they've connected to.