Which ink does the Epson WorkForce Pro WF-7820 use?

The Epson WorkForce Pro WF-7820 uses Epson 812 and Epson 812XL.

No. The Epson WorkForce Pro WF-7820 is an inkjet, so it prints with liquid ink from a cartridge, not toner powder, and no toner cartridge fits it. If you were searching for WorkForce Pro WF-7820 toner, the part you want is Epson 812 and Epson 812XL.

Yes. Besides the standard cartridges it accepts Epson 812XL. A high-yield XL holds more ink and costs less per page, which is worth it if you print often.

Epson can stop third-party ink working here with a firmware update, and it is the update rather than the cartridge that does it. Epson's own guide for the WorkForce Pro WF-7820 says so in as many words: cartridges that functioned before a firmware update may not continue to function. Epson gives you the warning in advance, which is little use once the printer updates itself, so the defence is to decide when it updates. Turn automatic updates off before you fit a compatible, and keep a genuine cartridge in reserve for the day you do have to update.

Rated yields are the Epson 812 at about 300 pages, the Epson 812 at about 350 pages and the Epson 812XL at about 1100 pages. Real-world yield depends on how much you print and how much ink each page uses.

By list price the lowest cost per page here is the Epson 812XL at about 3.4 cents per page. A reliable compatible or remanufactured cartridge can lower that further.