An address in the static residential catalogue is registered to a named internet provider in the country you pick and hosted on server infrastructure, and it stays yours for the period you paid for — it does not rotate, and it is not shared out of a pool between customers.
Each country in the first list shows the one-month price per IP it starts at. What matters more than the count is what sits under each flag: 21 distinct carriers in the United States alone, and one or two in most other countries. You choose the carrier yourself in the order form on the pricing page rather than having one assigned afterwards.
The other two lists work differently. Datacenter is covered in 71 countries and listed without prices. Mobile is covered in 13 countries, and each row shows the price its plans start from in the catalogue — a plan is a period of access, some with a traffic allowance — but mobile is not on sale yet, so those figures describe the catalogue rather than an order you can place. More on each: mobile proxies, datacenter proxies.
Countries and prices are read from the snapshot of the ProxyJam catalogue this site is built from, the same data the order card uses; the datacenter list is coverage for a product the catalogue does not sell yet, which is why it has no prices.
Who an address is registered to
Which network holds a block of addresses is public record. The regional internet registries, which receive their address pools from IANA and coordinate through the Number Resource Organization, record each registration. An RDAP query (RFC 9082) returns it, naming the organisation the block is registered to and a two-letter country code (RFC 9083), so you can check any address you hold yourself.