See this post and let us know if you have further questions. The link that David @DavidW gave to you is very informative, but it does not show you how extremely difficult it is to play the 10-hole blues harp in all 12 positions. Howard Levy is amazing and makes it all seem easy. But to play well in all 12 positions on one harp you need to master all bends, overblows and overdraws! That means not only being able to play them (accurately and easily) but also being able to incorporate them into the music fluidly. This is something that very few professionals can claim to do.
Regards,
– Slim 