Hi Suresh/Swamy
The planting distance depends to a great extent on the amount of training that you want to provide the plant. Lesser the height that you want it to grow, more pruning required, but after a while it becomes pretty easy.
We started off with banganapalli/kesar/dusheri in 20 X 20 spacing and had around 100-110 plants per acre. This year, we trimmed all of them out to 6 ft X 6ft and planted additional plants in between. We have around 13 acres set up in this fashion and another 7 acres with mixed varities of mangoes. There are currently about 48 varities, with another 24 expected in 2 months.The density now is at 680-700 plants per acre. You can also check out myfruitfarm.in, where another farmer from Ohio state, also has a high density alphonso plantation going on.
We are trying to convert all our practices to organic and natural farming.
Shiv