Here's a timelapse video of reader Christopher Cayco drawing all 807 current Pokémon. It took him 350 hours over a span of 42 days.
Actually, there are more than 807 Pokémon: while he's got all of them accounted for, he's drawn some of them more than once, so there's actually over 1100 pocket monsters in total on the final image.
You might remember something like this from 2016: that was also Christopher, and this new image is a result of him trying to add new Pokémon to the existing illustration, before deciding that his improved linework would be better suited to an all-new picture.
Here are some detail shots of the poster so you can see how much work has gone into each individual Pokemon, along with a low-res shot of the completed image. You can actually buy a print of it if you want, it's available here.