I was trying to figure out why
Serebii is one episode behind. Turns out that EP448 'Pokémon Ranger! Deoxys Crisis!' is listed as a single episode. It was broadcast as a one-hour special, but it is considered
two episodes in official listings.
PocketMonsters.net's count is indeed at 899 (including the delisted and unaired episodes, much like Serebii).
Dogasu seems to be doing his own thing, counting some (but not all) delisted episodes, some (but not all) specials, and some (but not all) unaired episodes. He has a point in some cases, but his method doesn't seem completely consistent.
Meanwhile, Bulbapedia's count is at
893 (a different page mentions
896), and some rather arbitrary reasoning is behind some episodes that don't have a code. Most recently,
an episode was named a special because someone didn't think a regular one could have a name like that.