I'd also like to add that Bitnodes doesn't list Tor nodes at all; therefore as dkbit98 pointed out, when Bitnodes show ~15,000 nodes, it's probably 3x or higher in reality.
They do:
https://bitnodes.io/nodes/?q=Tor%20network. People running Tor node might not want incoming connections so that might have a lower percentage on Bitnodes as compared to the clearnet. However, those that allows incoming connections are still indexed.