I’d like to point out that probably we are seeing just the ones that do not use TOR or other VPNs , and these are the modalities usually advised when you install certain nodes os like mynode. So in my opinion you need to take this number (20.000) and multiply at least by 5 to have a number closer to reality
Discussion
I'm not sure if the N⁰1 rank is the VPN/tor nodes, because it says n/a for the country 🤔
According to Bitnodes, there are approximately 20,663 reachable Bitcoin nodes. 
Research from 2021 using the Passive Announcement Listening (PAL) method estimated about 31,000 unreachable peers active per day at the end of 2020. 
Combining these figures suggests that the Bitcoin network consists of approximately 51,663 full nodes, with unreachable nodes constituting a significant portion.
In the context of Bitcoin full nodes, “unreachable” refers to nodes that are part of the Bitcoin network but do not accept incoming connections from other peers. These nodes can still actively participate in the network by initiating outbound connections to other reachable nodes, but they remain hidden for various reasons. Here’s a breakdown:
1. Network Address Translation (NAT):
• Many devices are behind NAT, where the router or firewall does not forward inbound traffic to the node.
2. Firewalls:
• Some nodes are configured with firewalls that block incoming connections for security or privacy reasons.
3. Tor or VPN Use:
• Nodes using the Tor network or VPNs often keep their IP addresses private, making them unreachable to the broader network.
4. Configuration Settings:
• Bitcoin Core and other implementations can be configured to disable incoming connections, limiting the node to outbound communication only.
5. Dynamic IPs or Private Subnets:
• Nodes with frequently changing IP addresses or those operating in private subnets may not be discoverable by others.
How Unreachable Nodes Contribute:
Unreachable nodes still download and verify blocks and transactions, contributing to the network’s decentralization and security by:
• Validating the blockchain independently.
• Relaying transactions and blocks to the reachable nodes they connect to.
However, they do not directly contribute to the propagation of new transactions or blocks beyond their connected peers.
Detecting Unreachable Nodes:
Detecting these nodes is difficult because they don’t publicly announce their presence. However, researchers estimate their numbers based on indirect observations, such as:
• Outbound traffic patterns.
• Passive monitoring techniques like the Passive Announcement Listening (PAL) method.
Thank you for that info!