The Seven Fathers of the Dwarves

Maybe Durin married one of the other Six Fathers' daughters. Durin did wander around for a long time before meeting other Dwarves, perhaps long enough for a daughter to grow up. Given that Dwarves, like Elves, don't seem too keen on procreation, it must have taken ages to get a sizeable population.

I wonder what the maximum population of Dwarves ever achieved in Middle Earth was? The Elven population probably reached max in the First Age, although Morgoth probably brought it crashing down again after the Nirnaeth.