Self-sustainable population size

[Amanita]

Sorry for the long absence. I haven't done much writing during the last months because I had plenty of non-fictional writing to do. I still do but I do spent a bit of time making stuff up again and I've reached a point where I'd like to get your opinion.
I'm in the process of creating a magical race which is supposed to be relatively small in number. (Or else they'd throw over the balance of the setting.) Yes, I got the feeling that I need a few more fantastic elements to get where I want.
Anyway, I'm wondering how large their number has to be if they're supposed to survive over a longer time period without (much) intermarriage with other groups. I'd like to have numbers between 20 000 and 50 000 individuals. Is that realistic or too small? Some things can be hand-waved with magic of course but I'd prefer to avoid this if possible.
I'm sure there has been done some research into this subject matter. I even believe I've read something about on here before but I'm not able to find it again.

 

[CupofJoe]

I don't speak to it's aptness for your situation but have a look at this:
Minimum species population to ensure adequate genetic diversity?

They are talking about animals but the figures and factors should still be roughly relevant... I would think that 20-50,000 is more than enough for long term stability...

 

[wordwalker]

Then again, that assumes their genetics are the same as ours: partial inbreeding is unsafe, genetic variety is key to disease resistance, and so on. The more magical the race, the less binding our numbers are.

 

[skip.knox]

Truly. Why worry about genetics? This is fantasy. What sort of realism are you aiming for, here? Economic? Political? Biological? Each has its own parameters and variables. And you can toss any one of them out the window if you want.