How many houses can you fit in 420 square meters of land?

In reality, this depends on the land use bylaw for the city/town/municpality that governs the land. Assuming this is a "standard" lot of 15m x 30m and is zoned as "residential", you can probably only have one house on the lot.

Legal stuff aside, I guess it depends on how big the house(s) would be. You could come up with an average size of a few houses (based on real estate Listings) and then just use division:

420m2 / ???m2 = # of houses that could fit