Affordable housing is typically apartment style, not houses on plots of land.
Twenty acres can actually a huge amount. There's some affordable housing near UCF that I use to live across from. It's on about three acres and has about 1,400 apartments on it (1-3 bedrooms). Based on that, I think we could underestimate about 400 apartments ("homes") per acre. At that, 20 acres can give you 8,000 apartments between 1-3 bedrooms. That can support between 15,000-20,000 people without breaking a sweat.
That's bananas, and definitely a step in the right direction. The mass transit hub will make it even more attractive.