Well, in the other thread where I talked about this last month, I gave several details, but I will clarify a couple things here that might answer your question:
1-50,000 **or less** :)
50K is a maximum, the maximum population that can be expected to:
2- Reorganize the town/city to adapt to the new circumstances. I talked about this aspect a bit more in the other thread (I forget the title, it was last month but I had a few lengthy posts in it with important details), but basically no, you cannot expect 50K, or anywhere near that number to all be preppers, you'd be lucky to have a tenth of that all be preppers. The point of a smallish town is that it's still small enough to change how it operates, they could reallocate people and skills and still have a prayer of feeding them (a prayer) by growing locally when supply lines and deliveries from farms stop. A 50K or smaller community is quite likely to be surrounded by farmland (if you follow my other location cretiria besides population) actually, or at least arable land. 50K can survive, with cooperation (there would be challenges) and reallocating jobs and having some folks change profession etc to more relevant occupations.
So basically, don't seek 50K, that's the max! That many people would not be preppers, but they could reorganize.
Limiting yourself to 100% preppers limits the size of the community, and what you gain in preperedness you might lose in defensibility, however well armed a community of 50 preppers might be, an 'army' of a couple hundred roving bikers would probably overrun them, especially once hostages were taken (family members).
You also limit yourself to a bunch of weirdos. You should meet more preppers. Some of them are fantastic people, totally normal and well-adjusted, some of them I wouldn't want near my family, I might take my chances with the paramilitary mobs :D