Ah, this is where your friendly neighborhood international cartoons sort of expert comes in handy...
A series like Pokemon which lasts for decades or so will ALWAYS amount a large number of movies. Doraemon has a whopping 30! Of course, Doraemon's a staple that's been around since the late 60's and has been on Japanese TV since 1979 with a disastrous 1973 series that lasted 6 months. In fact a lot of popular anime get several films...One Piece has just released movie number 10. Some only get one or two movie. It took a 1970's series Yatterman to FINALLY get a theatrical anime movie until it's 2008 revival to get a movie... oddly enough, premiering months after a live action remake movie. Some movies can be as short as 5 minutes, some are even around the 20-45 minute range, and shown as double or triple bills with other short films.
In the US this practice is exceedingly, and disappointingly rare. Some of these films signify (or cause) the drop in popularity of the characters... the Powerpuff Girls for example. Nickelodeon had successful runs with The Rugrats (giving them a whopping and unheard of in this country 3 theatrical movies) and Spongebob. And South Park and Beavis and Butthead had profitable theatrical release. Others aren't quite as lucky. I wish I saw Batman Mask of the Phantasm in theaters during its short run.
To give you a hint, if we had the same system of animated movies based on TV shows that Japan has... well, it wouldn't have took us almost 20 years for a Simpsons movie... I'll say that. And we'd be on our 15th Simpsons film by now.
As for Pokemon... not a fan! Not at all. But I'm not going into that. I'm shocked that that's still popular, even over there. Especially if you know your obscure earlier animes like I do, you'd see that it's a myriad of old tropes... the team of three bandits that are trying to steal Pikachu? Stolen from the Time Bokan series. I like them, but when you see all the Time Bokan series villains in action, you just can't even laugh at them anymore...