In my opinion it’s considerably less than people think, though there are certainly plenty of seedlings out there. The majority of mango trees that people plant are purchased, and nearly all those sold by nurseries are grafted. When surveying mango trees 10 years ago, I found that Most backyard trees fell under the old-school Florida mango camp (Haden predominant, with lots of Kents, Tommy Atkins, Keitt, Valencia Pride, and Carrie). Then you had your less commonly planted older grafted varieties (Glenn, Van Dyke etc), plus grafted “newer” varieties planted within the Mango Renaissance of the last 25 years.
Among seedlings, many are polyembryonic Turpentine, so a smaller number of the total trees are truly unique monoembryonic types. i would say this number may be less than 30%.