Seven: Four From Five or One?

[The Might]

Of the original possesors of the Rings we know very little.
According to Dwarf legend, the first of the Seven to be forged had already been given by the Elves to Durin III, so Sauron could not find it. But Sauron took the remaining six Rings and distributed them to other Dwarf lords.

Firstly, we don't even know who these lords were, though it is in my opinion a tempting idea to think they belonged to each of the other six houses, since this way, Sauron could corrupt more at once. But, of course, this is only speculation.

But the possessors of the Ring did not display it or speak of it, and they seldom surrendered it until near death, so that others did not know for certain where it was bestowed. Some thought that it had remained in Khazad-dûm, in the secret tombs of the kings, if they had not been discovered and plundered; but among the kindred of Durin's Heir it was believed (wrongly) that Thrór had worn it when he rashly returned there. What then had become of it they did not know.

As we see, it would have been hard to even know when who had these Rings, even if you belonged to his own folk.
It could be their possessors had wandered off driven by the need for more gold, more treasure and were lost in the wilderness and then attacked by Dragons.
I don't think we have to necessarily think of large battles against dragons, and if some did take place, they might have taken place in far away lands, as Legate already said.