I made a Mango Mead. My suggestion, start with a batch of show mead and put the mangos in the secondary. Mangos are really the devils fruit, as in hard to deal with. Basically you need to deskin them and then de-seed them. The seed clings to the flesh too, not like a peach.
I recomend the blanching method of removing the skins. Basically it's putting them in boiling water and for a few seconds and then in ice water right away and then using a paper towel or just a thin towel to rub the skin off. Be very careful taking out the seed at this point as it is quite slippery.
As far as amount, I'd go 15 pounds for a 5 gal batch, and in the secondary.