In Bitnodes you only see the nodes that are listening and that are visible.
There are many hidden nodes that are not visible, nodes that are not listened to, they can be behind a firewall, Tor or because they are configured not to listen to the connections.
Where you can see the highest number of active nodes is on the page of
https://luke.dashjr.org/programs/bitcoin/files/charts/software.htmlIt currently shows +/- 43,300 but it is estimated that there are many more hosted on servers or in the cloud. There are probably many, many more.
