I'm reading that IPv4 addresses consist of a network number and a host identifier. Is it accurate to say the Internet is a collection of networks, and this "network number" refers to just one of these networks? For example, a college campus might be such a network. Or a city-wide ISP might be another network.
Version 4 of the Internet Protocol (IPv4) defines an IP address as a 32-bit number. However, because of the growth of the Internet and the depletion of available IPv4 addresses, a new version of IP (IPv6), using 128 bits for the IP address, was developed in 1995, and standardized as RFC 2460 in 1998. IPv6 deployment has been ongoing since the mid-2000s.
A public IP address, in common parlance, is a globally routable unicast IP address, meaning that the address is not an address reserved for use in private networks, such as those reserved by RFC 1918, or the various IPv6 address formats of local scope or site-local scope, for example for link-local addressing. Public IP addresses may be used for communication between hosts on the global Internet.
So in accordance to your question, a public IPv4 network number could be an ISP, where the host identifier, could be a customer with an internet connection rented from that specific ISP.
Companies can also own public IPv4 networks, where the single public IP addresses within those networks are the host identifiers.