Using the Same Harp

You've run up against one of the realities of the diatonic harmonica. People play songs in more than one key, but each harmonica is designed to play in only one key, so to be in the game you need harmonicas in several keys.

It is possible to hear something played in another key on a different key of harmonica, and then figure out how to play the same pattern using a completely different set of notes on a C-harp, but this is a sophisticated skill that will make your learning curve much steeper.

You say you don't plan to buy harmonicas in more than one key. I'm wondering why.

If you simply don't want to commit to more harmonicas until you know that you want go all the way into playing blues harmonica, this is a self-deating approach. It's like saying you want to play baseball and buying a bat but refusing to buy a ball or a glove.

If you simply can't afford to sink cash into several Special 20s at something like $30 each, then one alternative would be to buy a prepackaged set of inexpensive harps in several popular keys, such as the Hohner Piedmont Blues. This at least gives you the keys you'll need. The harps aren't any great shakes - you'll definitely notice the difference, but you'll at least have the tools you need. Later, as money comes available, you can upgrade to better harps one at a time.

if you feel that you should be able to play in all keys on a C harmonica, that isn't how blues harmonica works. At least not all the way. Most of the blues you'll hear played on a C harmonica will be in the key of G (or second position). Some of what you hear will be played in D (third position) or in C (first position).

But let's take that example of a song played in D. The most common harmonica key for playing in D is a G-harp. And the things you can do on a G-harp sound different from what you can do on a C-harp - each is unique, and one can't duplicate the other. So you need that G-harp to make the characteristic blues sounds that you hear when the great players lay down classic harmonica plicks in the key of D. For more on this, read up on harmonica positions, and listen to examples of players where the key and position of harmoncia are known. After awhile you'll start to pick up on the unique character of each and appreciate why you need different keys of harmo to make those sounds.

As for using a C-harp to play in the keys of, say, E or A, it's possible but requires an advanced level of technique and is a path that used in jazz, requiring an approach completely different from that employed in the blues idiom.

You can find books, such as my "Blues Harmonica For Dummies," that allow you to do everything on a C harmonica. But the rich content on this site is huge treasure or you to enjoy and learn from, and if a few different keys of harmonica are needed to unlock the treasure chest, I'd say that those keys are worth the price of admission.

What Winslow is writing is of course true!
What Hank is writing is a nice workaround for the time being, at least until you're convinced that harmonica is THE instrument for you AND if you want the play blues music.
For most of the lessons on this site you'll need only a few different keys: C, A, G, D.
If I recall correctly only these keys are used in the beginners lessons which can keep you busy for a year.
And keep in mind, for the price of 1 gitar, 1 piano, 1 trumpet, 1 'whatever' you can buy lots of harmonica's!