A lot of the leaves on my mango tree (Tampa, FL) have these black spots.
My google efforts have identified Bacterial Black Spot as the leading suspect, but I'd appreciate any knowledgeable second opinions.
Meanwhile, some of the mangoes have black areas, as well. Could also be BBS from my reading, but not exactly sure. I have mostly seen this on mangoes that have fallen, not ones still on the tree.
Mango anthrachnose is a common disease. There aren't a lot of things you can do with a big tree, you have more options with a small one. Good sanitation is important to keep the spores from spreading. Pruning to open up the canopy so the leaves dry and if the tree is small enough you can use preventive anti fungal sprays.
https://www.ctahr.hawaii.edu/oc/freepubs/pdf/pd-48.pdf
https://www.ctahr.hawaii.edu/oc/freepubs/pdf/pd-48.pdf