I would have a couple dozen personal favorites, and the rest of a single variety, that can be marketed commercially. in commercial operations, the last thing you want is, multiple harvests, labor is expensive, unless you plan on doing all the work yourself. picking, cleaning, sorting, and packing, mangoes from 250 trees. is a lot of work.
commercial operations, usually only have 1-3 varieties, in the D.R, they use potassium nitrate to trigger flowering ( synchronize ) the goal is to get all the flowering at the same time, so you don't have fruits in all stages of development.
harvesting is typically done in three stages. the early fruit, are picked by lot hands. and sold locally to markets. the main harvest is done next, this is their main export, they hire hands to get things moving fast,
the last mangoes, are sold to street vendors, they come and pick them, and clean off the tree of whatever was left over.
300 trees is a difficult number, its way too much for a single person to handle unless you have a lot of time on your hands, and not very profitable as a business if you have to hire people.