getpeerinfo returns information about currently connected peers, but not the entire network. Since Bitcoin is a decentralised peer-to-peer network, no peer can possibly establish an accurate view of the full network connection graph.
Consider how these peer connections are established which are returned by
getpeerinfo. When your node is started, it will bootstrap a list of peer endpoint addresses from known seed servers. Once a sufficient number of addresses is acquired, it begins connections (This address list is actively amended as new addresses are propagated to your node after the seed session).
Outbound connections: Up to N peers on address list acquired during the seed session.
Inbound connections: Accepts connections from up to M peers connecting to your node
Manual connections: Configured permanent peers your node (attempts) to maintain connections to.
There is a configurable limit to all types of connections for your node. In any case, the limited view of the network means your node will never have a full view of the global Bitcoin p2p network.