I agree with Dusty.

When I said I have different keys covered, it's because I have standard dulcimers that I mostly tune to D; a baritone that I mostly tune to A; and a Ginger (small dulcimer) that I mostly tune to G. Of course, any of them can be retuned as needed. So I like having several different types of dulcimers (voices). Also, the McSpaddens are the traditional, less-deep style; 2 have galax backs and are deeper bodied; and one is in-between (and also has a pickup installed for small gigs). So like Dusty, what and where I'm playing often dictates which dulcimer I play. Two dulcimers usually accompany me to festivals.

First, let's keep things simple. 

The most common keys in old timey, folk, or bluegrass jams are C, D, G, and A.  With a standard-sized dulcimer tuned to an open D (DAA or DAd), you can obviously play in the key of D.  You can capo at 3 to play in G or capo at 4 to play in A.  And you can tune down to CGc or CGG to play in C.  (And there are other ways to get those keys as well, without switching instruments.  You could, for example, tune DGd to play in G, although if you chord and/or fret across all the strings that tuning will require some adjustments.)

So for 90 percent of your playing--even in multi-instrument jams--you only need one dulcimer.

However, I will be honest that I often bring 2 dulcimers to jams.  One a standard dulcimer and one in the baritone range.  The standard size dulcimer can usually be tuned anywhere between C and E.  The baritone can usually be tuned anywhere between F and A.  If we add the half steps in between those ranges, we get 10 keys, the only ones missing are B and Bb. And you can finagle (is that how you spell that word?) around to get those keys as well. For example. if you tune a standard dulcimer up a half step to Eb, you can capo at 4 to get Bb.

So there is no need to get multiple dulcimers for multiple tunings and multiple keys.  Even without using a capo, you can get 10 of the 12 possible keys with dulcimers in two tonal ranges. (One of my baritones can get up to Bb as well, but be careful and wear protective goggles if you try that on your own.)

However, I have several dulcimers.  Why?  I find different kinds of playing sounds better on different kinds of dulcimers.  I love my Blue Lion for fingerpicking, but don't think it sounds so hot flatpicking. I love my Modern Mountain Dulcimer for flatpicking, but find the action is too low and the strings too close together for fingerpicking.  I have one dulcimer that originally came with six strings, and it sounds great on those tunes with tons of strumming.  But playing more nuanced arrangements with lots of precise picking and lots of hammer-ons and pull-offs, those double strings just don't do the job.

So don't go buying different dulcimers for different tunings or different keys.  It's completely unnecessary. However, you may find that as your playing evolves, you develop different preferences and one or another dulcimer may be great for one tune played in a certain style but not for other tunes.  Getting multiple dulcimers to please our preferences, though, is a luxury, not a necessity.  

The first OLD PAL music festival I went to in Palestine, Texas 4 years ago one guy said to me..."the key of D is the same in Dad or Daa, so just play your noter style and we'll do our chording"   What a wonderful time I had.  That was my first dulcimer festival.  I sure hope we can meet in 2021.  aloha, irene

When i used to go to old-time music festivals and gatherings a lot, I'd often bring two dulcimers or two banjos:

One dulcimer I'd use for the keys of C and D (tuning up or down one step on all strings), and the other dulcimer I'd use for the keys of G and A (tuning up or down one step on all strings).

I'd often do the same with the two banjos.
Why?  Because going between the lowest key (G) and the highest key (D) sometimes means retuning a string by four whole steps up or down, and that's a real big jump which can result in more stressed and broken strings if you do it often. It 'can' be done, but is frustrating to do back and forth in the setting of jams or if folks change keys too often. The alternative was to use capos, which I try to avoid because I dislike the sound and feel.

Sometimes, to avoid bringing four instruments, I'd bring a dulcimer for C and D and a banjo tuned for G and A.

One minor point-  I'd like to mention that beginners often think that DAd is a completely different animal than CGc. Or that DAA is a completely different animal than CGG. They balk at having to 'learn a different tuning' and all that entails. They don't realize that it's really the same tuning but simply one whole step up or down, and it will play in the keys of C or D. That means they can use all the same fingerings and frets in the key of C as they are already familiar with in the key of D. They can follow the same Tab numbers, and don't have to learn new stuff. They'll just be playing in the key of D or C.  Same is true for the (1-5-8) tuning pair of DAd and CGc. If you are comfortable playing in DAd you already know how to play tunes in CGc... Likewise for the 'pair' of (1-5-5) tunings DAA and CGG.

I have several tuned DAA, a few more tuned DAd, and at least one tuned CGG. Those are the tuning I use most of the time, but I do retune for other tunings. I think I currently have 12 mountain dulcimers and four under construction; two of which I will keep. I also have four PA German zitters.

Ken

"The dulcimer sings a sweet song."

I'm guilty too. But hey, it keeps our passion fun and exciting. I've also changed dulcimers when my interests have changed. I've met some wonderful people through the sale of outgoing dulcimers. I have to admit though, I'm pretty happy with what I have right now. With my group, I have different keys covered, but also acoustic amplification and travel. Since my first (and longest) passion is fingerstyle guitar, I have a whole other "problem" (GAS). I have 5 dulcimers and 7 guitars.

Let's see... 7 dulcemores and 1 dulcimer; in two different keys and four tunings.

yep, many of us have more then one dulcimer.   I make them and sell them sometimes.   I also have fun finding them all sad and neglected in some antique store or elsewhere.   I bring 'em home and fix 'em up and loan 'em out to folks that would like to learn.   soooooooooooo fun.   So yep.  I have them tuned in Daa and Cgg and minor tunings.  It's fun to have more then one.   good excuse to look for another one for you.   oooooooooooooops, aloha, irene

I have my dulcimers tuned to DAd DAdd. So I find a song that sounds better in DAaa, and then there is one in CGcc. Tune, retune, and retune again. How many have dulcimers just tuned to different keys and use that when they find a song? I do have capos but only use them for one song in a set if it has another key.