I cant predict a number, but since they keep remaking the games over and over all the time, next being a silver/gold remake "i think" i predict there will be probly 800 or so pokemon.
Tbh tho i doubt the series will ever end, the Pokemon name has been out since before some you were born that or when you were still in diapers :P yea that long and its still very popular, the card game has made a comback and with the release of D/P and how well its done specially with the Introduction of WI-FI trading and battle's i doubt the series will ever end.
They wont stop until it stop's making money. Pokemon is like the Mario of my Generation it isnt gonna end.
As for the guy who said to stop making pokemon and make a MMO that would never work. Alot of people like Pokemon because of the Handheld games. The way the game is played how simple and easy it is the game has been played the same way for 11-12 years now and thats why people keep comming back. Whats turns me away from pokemon are crappy games like the ones for the Console's with all the fancy graphics and the wierd gameplay. I'll stick the old fation way, sometimes good graphics and stuff like that isnt the way to go it turns people away.
A pokemon MMO would fail. It would be neat to try and actually walk around like you would on say world of warcraft catching pokemon in a free huge world, training and battling with others online but i just dont think it would work it isnt the same