But there is also a house, pool, driveway on that lot.
Most people seem to be forgetting that aspect of the equation. Other factors such as shape of the lot, planting restrictions in the front yard, presence of things like a septic tank, etc. can all affect the # of trees allowable. Either way, I'm with the majority opinion that 250 mango trees on 1/2 acre seems implausible.
We have approx .4 acres here with the benefit of a pie-shaped lot that allows for extra planting room. We're probably in the 40 - 50 tree range with arguable space for a couple more (if and when the kids' playset is removed), and even that is difficult to manage/keep pruned effectively.